How to learn Gaulish language?

Study with an expert historian, archaeologist, or Linguistics scholar who specializes in that area.

Or, you could locate a text by a Linguistics scholar that provides you with the basic grammatical rules and a lexicon of as many words as are known. The language is primarily known from inscriptions, which seemingly have all been translated, so there wouldn't be that much for you to read.

Seriously, the best way to learn an ancient dead language -- especially one far less known than Latin or ancient Greek -- is to study at a university with a scholar who has a lot of experience with it.

Bring back Julius Caesar from the dead, and get him to make a revived Gaulish prince teach you.

Get in touch with Asterix and Obelix. It seems that there are only scraps of the language left in various inscriptions. See Wikipedia.