Caesar vs the Gauls might be a bit uneven, but it wasn't a foregone conclusion that Caesar would conquer the whole territory. The Romans had been sending governors north for years, and had long since gotten used to the Gauls being an independent peoples. Caesar was initially planning to campaign in Illyria, another province assigned to him at the time, but the migration of the Helvetii and then Ariovistus's aggression against the Aedui drew him into Gaul. It all worked out well for him, of course, but like Philip he far exceeded popular expectations. It's not as if it was just Caesar vs the Gauls at the time anyways; the Romans had numerous allies and the game provides plenty of opportunities for diplomacy. Caesar eventually turned almost all of Rome's allies in Gaul against him at one point or another, so we'll have that to live up to.