In theory some Galatians served in phalangitai capacities, just as some Jews and others did as well but they were all minorities of larger units, so there never really was a 'Galatian Phalanx Batallion' or whatever you'd want to call it. And, as far as I know even those who did serve as the occasional phalangitai were settled, or even born in Egypt. Its not like anyone went back and then the Galatian Kings adopted Macedonian tactics all of the sudden.

Now a case could be made for Galatian Immitation legionaires, which did infact exist though much later than the actual start date of the game.