There is a wealth of information available on the gods of the insular Celts, one of the many benefits of having a surviving literature. It can be more difficult to find information about the gods of the continental tribes. Here are a few links to explore.
Excellent site about Gaulish gods and religion. My link is to the English-language version of the site, but there is also (and, I believe, primarily) a French version. Good info, good citations.
Celtnet is a wonderful trove of information about Welsh, Gaulish and Brythonic deities, well cited and highly recommended.
Gods of Gaul and Britain Map
My own no-guarantees, very much a work in progress Google map that attempts to place those Celtic and Germanic gods of Gaul and Britain who we know primarily from archaeological evidence. If you see any glaring errors, please let me know. :)
If you know of any other good Gaulish links, please let me know!