Since the Huns and Egyptians arrived on the scene, there has been pretty consistent talk that Gauls aren't very good anymore. Some have said that trappers need to be removed and replaced with another special building. I think I recently saw a post here saying the Gaul troops should be cheaper. I think the best option would be a little different.
If a Gaul traps enemy troops in the trapper, they can convert trapped troops into Phalanx for half the price of the normal cost of Phalanx (conversion would be crop consumption; Example: one trapped Equites Imperatoris would equal 3 phalanx that can be built at half price). Those Phalanx build in the barracks as if they were normally built there.
Maybe if you have a Trapper level 15, Druid Riders would be allowed. And then at level 20 -- Haeduans. Allied troops would not be eligible for this process. Maybe slaves should be excluded altogether because of the opportunity for exploitation of the game.