If you play as Rome, it pays to take Spain and eliminate the Lusotannan before you even touch Gaul. Spain is richer anyway (if you play SPQR and tech up those provinces with deep mines, decent ports, and paved roads, you can make 50k mnai per turn from Hispania alone), and you need to stop the Carthies from expanding too much there.
In my recent SPQR campaign, I only hold Italy, the Mediterranean Islands, Spain, Africa, Egypt, and Southern France, and I'm making more than 100k mnai/turn. After defeating the Lusotannan, things got much easier.