After many hours of test play, I believe that the solution for balancing Gauls is rather straight forward.  Take away the slinger unit from the Britons in Age 1, give them skirmishers in the CC just like Gauls (make them advance to Age 2 to make slingers), and reduce all skirmishers attack from 16 pierce to 14 pierce.    Problem solved.  This can be patched in less than a week.  You're welcome.