I love them
First, I love every civ that has maluses in some way. Something they can't do that everyone else can. And Gauls fit this criteria.
Then, I love too their UD. Defensive and production? I love this!
Mines are also kind of OP but in th good sense. Each mine give culture, cultural bomb. But he more interesting is adjacency bonuses for districts.
I have the feeling that you'll play the Gauls as you play Korea with its Seowon: widespread districts surrounded with mines. But it's far more interesting that Korea.
Overall a very, very good first impression.
Oh, and the music sounds so good too. Dunno about historicity, but I like it!
(And bonus: even if it's ahistorical, as a gay man, I like the fanservice with Ambiorix chest. Guilty as charge ^^)