As with the Illyrian, Ligurian and Thracian languages, the surviving corpus of Gallaecian is composed of isolated words and short sentences contained in local Latin inscriptions or glossed by classical authors, together with a number of names – anthroponyms, ethnonyms, theonyms, toponyms – contained in inscriptions, or surviving as the names of places, rivers or mountains. In addition, some isolated words of Celtic origin preserved in the present-day Romance languages of north-west Iberia, including Galician, Portuguese, Asturian and Leonese are likely to have been inherited from ancient Gallaecian.
Classical authors Pomponius Mela and Pliny the Elder wrote about the existence of Celtic and non-Celtic populations in Gallaecia and Lusitania, but several modern scholars have postulated Lusitanian and Gallaecian as a single archaic Celtic language. Others point to major unresolved problems for this hypothesis, such as the mutually incompatible phonetic features, most notably the proposed preservation of *p in Lusitanian and the inconsistent outcome of the vocalic liquid consonants, and therefore address Lusitanian as a non-Celtic language and not closely related to Gallaecian.
Some of the main characteristic of Gallaecian shared with Celtiberian and the other Celtic languages were (reconstructed forms are Proto-Celtic unless otherwise indicated):
Indo-European*-ps- and *-ks- became *-xs- and were then reduced to -s-: place name AVILIOBRIS from *Awil-yo-brix-s < Proto-Celtic *Awil-yo-brig-s 'Windy hill (fort)', modern place name Osmo (Cenlle, Osamo 928 AD) from *Uχsamo- 'the highest one'.
Original PIE*p has disappeared, having become a *φ sound before being lost completely: place names C(ASTELLO) OLCA from *φolkā- 'Overturned', C(ASTELLO) ERITAECO from *φerito- 'surrounded, enclosed', personal name ARCELTIUS, from *φari-kelt-y-os; place name C(ASTELLO) ERCORIOBRI, from *φeri-kor-y-o-brig-s 'Overshooting hillfort'; place name C(ASTELLO) LETIOBRI, from *φle-tyo-brig-s 'wide hillfort', or *φlei-to-brig-s 'grey hillfort'; place name Iria Flavia, from *φīweryā- (nominative *φīwerī) 'fertile' (feminine form, cf. Sanskrit feminine pīvari- "fat"); place name ONTONIA, from *φont-on- 'path'; personal name LATRONIUS, to *φlā-tro- 'place; trousers'; personal name ROTAMUS, to *φro-tamo- 'foremost'; modern place names Bama (Touro, Vama 912) to *uφamā- 'the lowest one, the bottom' (feminine form), Iñobre (Rianxo) to *φenyo-brix-s 'Hill (fort) by the water', Bendrade (Oza dos Ríos) to *Vindo-φrātem 'White fortress', and Baiordo (Coristanco) to *Bagyo-φritu-, where the second element is proto-Celtic for 'ford'. Galician-Portuguese appellative words leira 'flat patch of land' from *φlāryā,lavego 'plough' from *φlāw-aiko-,laxe/lage 'flagstone', from medieval lagena, from *φlagĭnā,rega and rego 'furrow' from *φrikā.
The frequent instances of preserved PIE /p/ are assigned by some authors, namely Carlos Búa and Jürgen Untermann, to a single and archaic Celtic language spoken in Gallaecia, Asturia and Lusitania, while others (Francisco Villar, Blanca María Prósper, Patrizia de Bernado Stempel, Jordán Colera) consider that they belong to a Lusitanian or Lusitanian-like dialect or group of dialects spoken in northern Iberia along with (but different from) Western Hispano-Celtic:
in Galicia: divinity names and epithets PARALIOMEGO, PARAMAECO, POEMANAE, PROENETIAEGO, PROINETIE, PEMANEIECO, PAMUDENO, MEPLUCEECO; place names Lapatia, Paramo, Pantiñobre if from *palanti-nyo-brig-s (Búa); Galician-Portuguese appellative words lapa 'stone, rock' (cfr. Lat. lapis) and pala 'stone cavity', from *palla from *plh-sa (cfr. Germ. fels, O.Ir. All).
in Asturias the ethnic name Paesici; personal names PENTIUS, PROGENEI; divinity name PECE PARAMECO; in León and Bragança place names PAEMEIOBRIGENSE, Campo Paramo, Petavonium.
in other northwestern areas: place names Pallantia, Pintia, Segontia Paramica; ethnic name Pelendones.
Indo-European sonorants between vowels, *n̥, and *m̥ have become an, am; *r̥, and *l̥ have become ri, li: place name Brigantia from *brig-ant-yā < Proto-Celtic *br̥g-n̥t-y-ā < post-Proto-Indo-European (post-PIE) *bʰr̥gʰ-n̥t-y-ā 'The towering one, the high one'; modern place names in Portugal and Galicia Braga, Bragança, Berganzo, Berganciños, Bergaña; ancient place names AOBRIGA, CALIABRIGA, CALAMBRIGA, CONIMBRIGA, CORUMBRIGA, MIROBRIGA, NEMETOBRIGA, COELIOBRIGA, TALABRIGA with second element *brigā < Proto-Celtic *br̥g-ā < post-PIE*bʰr̥gʰ-ā 'high place', and AVILIOBRIS, MIOBRI, AGUBRI with second element *bris < *brix-s < Proto-Celtic *brig-s < *br̥g-s < PIE*bʰr̥gʰ-s 'hill (fort)'; cf. English cognate borough < Old Englishburg "fort" < Proto-Germanic*burg-s < PIE*bʰr̥gʰ-s.
Reduction of diphthong *ei to ē: theonym DEVORI, from *dēwo-rīg-ē < Proto-Celtic *deiwo-rēg-ei 'To the king of the gods'.
Lenition of *m in the group *-mnV- to -unV-: ARIOUNIS MINCOSEGAECIS, dative form from *ar-yo-uno-*menekko-seg-āk-yo- 'To the (deities of the) fields of the many crops' < Proto-Celtic *ar-yo-mno- ... .
Assimilation *p .. kʷ > *kʷ .. kʷ: tribe name Querquerni from *kʷerkʷ- < PIE*perkʷ- 'oak, tree'. Although this name has also been interpreted as Lusitanian by B. M. Prósper, she proposed recently for that language a *p .. kʷ > *kʷ .. kʷ > *p .. p assimilation.
Reduction of diphthong *ew to *ow, and eventually to ō: personal names TOUTONUS / TOTONUS 'of the people' from *tout- 'nation, tribe' < PIE*teut-; personal names CLOUTIUS 'famous', but VESUCLOTI 'having good fame' < Proto-Celtic *Kleut-y-os, *Wesu-kleut(-y)-os; CASTELLO LOUCIOCELO < PIE *leuk- 'bright'. In Celtiberian the forms toutinikum/totinikum show the same process.
Superlatives in -is(s)amo: place names BERISAMO < *Berg-isamo- 'The highest one', SESMACA < *Seg-isamā-kā 'The strongest one, the most victorious one'. The same etymology has been proposed for the modern place names Sésamo (Culleredo) and Sísamo (Carballo), from *Segisamo-; modern place name Méixamo from Magisamo- 'the largest one'.
Syncope (loss) of unstressed vowels in the vicinity of liquid consonants: CASTELLO DURBEDE, if from *dūro-bedo-.
Reduction of Proto-Celtic *χt cluster to Hispano-Celtic *t: personal names AMBATUS, from Celtic *ambi-aχtos, PENTIUS < *kwenχto- 'fifth'.
Some characteristics of this language not shared by Celtiberian:
In contact with *e or *i, intervocalic consonant*-g- tends to disappear: theonym DEVORI from *dēworīgē 'To the king of the gods'; adjective derived of a place name SESMACAE < *Seg-isamā-kā 'The strongest one, the most victorious one'; personal names MEIDUENUS < *Medu-genos 'Born of mead', CATUENUS < *Katu-genos 'Born of the fight'; inscription NIMIDI FIDUENEARUM HIC < *widu-gen-yā. But Celtiberian place name SEGISAMA and personal name mezukenos show preservation of /g/.
*-lw- and *-rw- become -lβ-, -rβ- (as in Irish): MARTI TARBUCELI < *tarwo-okel- 'To Mars of the Hill of the Bull', but Celtiberian TARVODURESCA.
Late preservation of *(-)φl- which becomes (-)βl- and only later is reduced to a simple (-)l- sound: place names BLETISAM(AM), BLETIS(AMA), modern Ledesma (Boqueixón) < *φlet-isamā 'widest'; BLANIOBRENSI, medieval Laniobre < *φlān-yo-brigs 'hillfort on the plain'. But Celtiberian place name Letaisama.
*wl- is maintained: VLANA < PIE*wl̥Hn-eh₂ 'wool', while Celtiberian has l-: launi < PIE*wl̥H-mn-ih₂ 'woolly' (?).
Sometimes *wo- appears as wa-: VACORIA < *(d)wo-kor-yo- 'who has two armies', VAGABROBENDAM < *uφo-gabro-bendā 'lower goat mountain' (see above).
Dative plural ending -bo < PIE*bʰo, while Celtiberian had -bos: LUGOUBU/LUCUBO 'To (the three gods) Lug'.
Under the P/Q Celtic hypothesis, Gallaecian appears to be a Q-Celtic language, as evidenced by the following occurrences in local inscriptions: ARQVI, ARCVIVS, ARQVIENOBO, ARQVIENI[S], ARQVIVS, all probably from IE Paleo-Hispanic *arkʷios 'archer, bowman', retaining proto-Celtic*kʷ. It is also noteworthy the ethnonyms Equaesi ( < PIE *ek̂wos 'horse'), a people from southern Gallaecia, and the Querquerni ( < *perkʷ- 'oak'). Nevertheless, some old toponyms and ethnonyms, and some modern toponyms, have been interpreted as showing kw / kʷ > p: Pantiñobre (Arzúa, composite of *kʷantin-yo- '(of the) valley' and *brix-s 'hill(fort)') and Pezobre (Santiso, from *kweityo-bris), ethnonym COPORI "the Bakers" from *pokwero- 'to cook', old place names Pintia, in Galicia and among the Vaccei, from PIE *penkwtó- > Celtic *kwenχto- 'fifth'.
Some local Roman inscriptions incorporating autochthonous names, appellatives, and phrases
Anthropomorphic stele with Latin inscription, and local anthroponyms (from Verín, Ourense, Galicia): LATRONIUS CELTIATI F(ilius) H(ic) S(itus) E(st)
In the 19th century a group of Romantic and Nationalist writers and scholars, among them Eduardo Pondal and Manuel Murguía, led a Celtic revival initially based on the historical testimonies of ancient Roman and Greek authors (Pomponius Mela, Pliny the Elder, Strabo and Ptolemy), who wrote about the Celtic peoples who inhabited Galicia;  there is currently a revival movement within Galicia (Spain) which often extends into Asturias, North Portugal and Sometimes Cantabria funded by the Celtic League in Galicia, this movement is championed by people like Vincent F. Pintado, Founder of the Gallaecian Language Revival Movement, Member of the United Celtic Nations, Sponsor of the Gallaecian Celtic League, Author of the Old Celtic Dictionary, However it is worth noting whether or not this is a legitimate language revitalisation project or a conlanging project.
^"In the northwest of the Iberian Peninsula, and more specifically between the west and north Atlantic coasts and an imaginary line running north-south and linking Oviedo and Merida, there is a corpus of Latin inscriptions with particular characteristics of its own. This corpus contains some linguistic features that are clearly Celtic and others that in our opinion are not Celtic. The former we shall group, for the moment, under the label northwestern Hispano-Celtic. The latter are the same features found in well-documented contemporary inscriptions in the region occupied by the Lusitanians, and therefore belonging to the variety known as LUSITANIAN, or more broadly as GALLO-LUSITANIAN. As we have already said, we do not consider this variety to belong to the Celtic language family." Jordán Colera 2007: p.750
^Prósper, Blanca María (2002). Lenguas y religiones prerromanas del occidente de la península ibérica. Ediciones Universidad de Salamanca. pp. 422–427. ISBN84-7800-818-7.
^Prósper, B.M. (2005). Estudios sobre la fonética y la morfología de la lengua celtibérica in Vascos, celtas e indoeuropeos. Genes y lenguas (coauthored with Villar, Francisco). Ediciones Universidad de Salamanca, pp. 333–350. ISBN84-7800-530-7.
^Galician words such as crica ('vulva, ribbon'), from proto-Celtic *kīkwā ('furrow'), laxe ('stone slab') from proto-Celtic *φlagēnā ('broad spearhead'), leira ('patch, field') from proto-Celtic *φlāryo- ('floor'), and alboio ('shed, pen') from proto-Celtic *φare-bowyo- ('around-cows').
^Among them the Praestamarci, Supertamarci, Nerii, Artabri, and in general all people living by the seashore except for the Grovi of southern Galicia and northern Portugal: 'Totam Celtici colunt, sed a Durio ad flexum Grovi, fluuntque per eos Avo, Celadus, Nebis, Minius et cui oblivionis cognomen est Limia. Flexus ipse Lambriacam urbem amplexus recipit fluvios Laeron et Ullam. Partem quae prominet Praesamarchi habitant, perque eos Tamaris et Sars flumina non longe orta decurrunt, Tamaris secundum Ebora portum, Sars iuxta turrem Augusti titulo memorabilem. Cetera super Tamarici Nerique incolunt in eo tractu ultimi. Hactenus enim ad occidentem versa litora pertinent. Deinde ad septentriones toto latere terra convertitur a Celtico promunturio ad Pyrenaeum usque. Perpetua eius ora, nisi ubi modici recessus ac parva promunturia sunt, ad Cantabros paene recta est. In ea primum Artabri sunt etiamnum Celticae gentis, deinde Astyres.', Pomponius Mela, Chorographia, III.7–9.
^Prósper, Blanca María. "El topónimo hispano–celta Bletisama: Una aproximación desde la lingüística". In: I. Sastre y F. J. Sánchez Palencia (eds.). El bronce de Pino del Oro Valladolid. 2010. pp. 217–23.
Búa, Carlos (2007) O Thesaurus Paleocallaecus, in Kremer, Dieter, ed. (2007). Onomástica galega : con especial consideración da situación prerromana : actas do primeiro Coloquio de Trier 19 e 20 de maio de 2006. Santiago de Compostela: Universidade de Santiago de Compostela. ISBN978-84-9750-794-3.
DCECH = Coromines, Joan (2012). Diccionario crítico etimológico castellano e hispánico. Madrid: Gredos. ISBN978-84-249-3654-9.
Delamarre, Xavier (2012). Noms de lieux celtiques de l'Europe ancienne (−500 / +500): dictionnaire. Arles: Errance. ISBN978-2-87772-483-8.
Jordán Cólera, Carlos (March 16, 2007). "Celtiberian"(PDF). E-Keltoi. 6. Archived from the original(PDF) on 24 June 2011. Retrieved 16 June 2010.
Koch, John T. (2011). Tartessian 2 : The inscription of Mesas do Castelinho ro and the verbal complex preliminaries to historical phonology. Aberystwyth: University of Wales, Centre for Advanced Welsh and Celtic Studies. ISBN978-1-907029-07-3.
Vallejo Ruiz, José María (2005). Antroponimia indígena de la Lusitania romana. Vitoria-Gasteiz: Univ. del País Vasco [u.a.] ISBN8483737469.
Wodtko, Dagmar S. (2010) The Problem of Lusitanian, in Cunliffe, Barry, and John T. Koch (eds.) (2010). Celtic from the West. Oxford, UK: Oxbow books. ISBN978-1-84217-475-3. ((cite book)): |first= has generic name (help)CS1 maint: multiple names: authors list (link)