Presently civs are arranged alphabetically. But gauls and britons are both Celts, and much of their strategy, such as build order, pertains to Celts, rather than Gauls or Britons. For example, as Celts they enjoy the bonus housing for many structures, most their buildings build quite a bit faster, they have a farming bonus, have small houses (good for walls), and Phase I skirmisher cavalry, phase II sword cavalry. They suffer from purely wooden barracks (I consider a partly stone barracks a bonus as it relieves wood pressure) and fairly abysmal Phase III which mean it's probably better to win the game in Phase II, although the ram is not a bad addition to their phase II units as it permits breaking open turtles. They basically have one huge difference. Britons get the Celtic Slinger which costs stone, and Gauls get the Skirmisher which costs wood. This allows Britons to use their starting stone to train a few slingers without increasing wood pressure, this is ideal for teching as fast as possible. Gauls on the other hand can train skirmishers in abundance on a purely wood and food income which is good if you want to spend longer at Phase I. At Phase II this difference disappears as they both have access to both units. They have some trifling differences as well, Britons which get war dogs and Gauls get naked fanatics. I'm not sure though if either of those units are of any great value. For Britons puppies can be produced using food alone and don't eat into housing - that's interesting, but do they change the outcome of a battle? Naked fanatics are a more normal unit. Neither unit contributes to economy so are really just things to dump resources into once you no longer care about cost-effectiveness. I imagine the Celtic cavalry will be game-deciding in competitive matches anyway, the only thing Celts could wish for is a cavalry archer, but their cavalry options are nearly as good as it gets and they can produce them quicker than anyone thanks to their great economy.