Aren't you forgetting the bit (helpfully quoted by someone) where Caesar talks about the Arverni, Aedui and some other major tribe (at the least) having had a major war of supremacy, with Ariovist and his boys having been brought in to break the stalemate that existed or had developed between the sides - and the German mercenaries then developing funny ambitious ideas of their own and turning on their erstwhile employers ?

If you ask me that speaks of a fairly severe level of conflict that cannot have done good for the numbers of capable Gallic soldiery available, given that the defeat of whoever it now was who ended up losing was doubtless rather bloody, and the way Ariovist could run amuck on his onetime allies and employers (and defeating a coalition gathered to get rid of him piecemeal on the side) doesn't exactly suggest the losses were being made good very swiftly. Enter Caesar and the additional drain he brought on the manpower pool (both through recruiting and enlisting, and as battle casualties both for and against) and I don't really see where exactly the major Gallic powers would have had ample opportunity to regenerate their pool of fully competent fighting men, doubly so given the somewhat time-consuming nature of the process in the Celtic "heroic" system.

All he says of the Helvetii was that they were skilled, he doesn't differentiate between cavalry or footmen when he says this.
Fair enough, but I think we can safely assume the Alps did not produce great numbers of capable horse. Unlike some other highland regions that area has AFAIK never been very good "horse country".