I personally I'm against going into Gaul early on as Spain.

In my first Spanish campaign, I kept at the Gauls and pushed into Lemonum and Narbo Martius with armies. However, I just get hit by endless stacks of enemy warbands. The towns I take aren't worth the effort.

Sure, I get heroic victory after another but warbands in melee power > iberian inf. I can only produce town militia at town level which gets slaughtered if left alone even for a little while. Gave up on it out of sheer boredom of fighting 2-3 boring battles every turn against 4-6 warbands.

Second campaign, I united Iberia immediately, then smashed a Gallic invasion into my territory. Meanwhile, I pushed into Numidia at Tingi and then moved onto Cirta and Carthage in short succession. I built ports quickly in Scallabis and Carthago Nova. I sent a family member to smash a Julii landing at Osca and then took Palma with that army.

I just remained defensive against the Gauls and now they don't bother me much. I'm becoming filthy rich from the nice territories that I grabbed. I might sweep up rest of Africa and then go into Sicily. Spain invading from the Italy south is kinda funny but makes most sense in my situation.

You will indeed be the most unpopular faction on the planet, at war with no less than 7 factions at once but you can crush all of them and this is the best way to fuel your economy. They don't call this Total War for nothing.