I think it wasn't a particularly wise move to invade Africa. It has its weaknesses and its advantages, but I'm not going to go into them as its too late!
Obviously, you need to take Cisalpine Gaul: Mediolanium, Patavium. It shouldn't be too much of a problem, although I think you have left it about ten turns too late for it to be easy. Then Massilia, then Narbo. That much is pretty straightforward. Carry on sending that army you've got in Sicily to Palma; the Balearics aren't much use, but all provinces generate cash and its a useful strategic springboard into Spain. So, once you've got the Balearics, Send the army there to Corduba and an army at Narbo to attack Osca. I won't detail the rest of what I'd do in Iberia; this is, after all, about dealing with Gaul, not Spain.
Don't ignore those northern provinces of gaul or someone else will get them; i.e., the people of the Woad Stag. They are a nightmare; believe me, they're my brothers. So, you should have two decent armies at Narbo when you invade Spain. Army1 goes to Osca and thence deeper into Iberia, and army2 goes North. Take Lemonum (Aquitania) then Condate (Armorica). Meanwhile, send a force from Massilia to Lugdunum. Capture it by starving them out, of possible. Then squash Alesia from both sides. Then send a reconnaissance force (town watch are fine) ahead of your main army to Samarobriva, with a spy. This is simply to draw out any ambuscades the Britons will almost certainly have planted. Then send at least a full stack army from Alesia after them, besiege and assault Samarobriva, and invade Britain quickly with as small a force as possible. Meanwhile, Spain should be mostly under control by now, along with Northwest Africa. The reason for invading Britain quickly is to stop the Britons getting a decent army at Londinium, which can cause serious problems. You should have plenty of money coming in by now.
Two good armies from Alesia and Samarobriva (which is usually at least a large town by now) should be able to hold your border against Germania from now on. Wait until Trier and Batavodurum are a satisfactory size (I know, it takes forever), then attack them. But never ever conquer and then abandon in RTW if you can help it; it's a sheer waste of time, finances, troops, and pretty much everything else you need to win in the game. Watch out for the huge stacks the Germans constantly leech out to send at Alesia and Lugdunum!
Sorry about the long post. I have a lot to say.
[This message has been edited by Edorix (edited 02-23-2009 @ 11:12 AM).]