Contentious! The existence of *gw in Proto-Brythonic is far from assured.Moose-tache wrote: ↑Sat Apr 11, 2020 5:16 pmThis is much like how kw later became p in Gallic/Welsh, but gw did not merge with the already extant b.
Those who favour the Insular Celtic hypothesis (i.e. that the Brythonic and Goidelic languages form a branch on the Celtic family tree distinct from the Continental Celtic languages), assume that Proto-Celtic *gw remained as such in "Proto-Insular Celtic", giving reflexes of g in Goidelic and gw in Brythonic. For example, PC *gwed-yo- 'to pray' (< PIE *gʷʰedʰ- 'wish') gives OI guidid and MnW gweddïo.
However, we have a further reflex of the same etymon in Gaulish: uediíu-, which certainly indicates an initial /w/. Given that initial *w in Brythonic has the reflex gw anyway, there is a large body of opinion that would group Brythonic with Gaulish here.
Personally, I suspect that if there was a gap in Proto-Brythonic where one would expect a voiced labiovelar stop, it would be a further motivating factor "pushing" *kw towards *p.