Essentially, we are playing the role of Pompey in Spain to Gemino's Sertorius - the materially and numerically superior force hamstrung by poor logistics facing a more mobile force specialising in ambush, with greater local knowledge and support, lead by a superior general. We could do considerably worse than follow Pompey's playbook. Get your logistics in order and play a cautious, methodical game - attrition favours you. Don't rush to a decision - avoiding defeat is more important, because the longer you do so, the more cracks appear in the other man's aura. The enemy commander can only be in one place at a time, so use your numbers to put him in a fork. Frustrate him, stress him out. Offer money and amnesty to traitors to ramp up the paranoia in the enemy camp.
To move from such generalities to the specificities of our situation, our first priority has to be food, both for the legion and Bovianum itself. Gemino knows this, and so his priority is to stop us feeding everyone. He can almost certainly counter any one attempt we make, so we need to make more than one. He doesn't have the strength to contest us everywhere. If he strikes at Aquilonia, it will be at a time and place we are expecting him, on ground we will have had a chance to prepare, and close enough to our base of operations that we can muster overwhelming force with relative ease. If he strikes at the hinterlands of Bovianum, he runs the risk of eroding his support base, and (in order to do so efficiently) he will have to disperse his men somewhat, allowing us to swallow up isolated groups. Finally, if he targets our supply convoys, he will be sacrificing the opportunity to interfere with our supplying Bovianum, which means that we can start sharing supplies next turn if absolutely necessary, and he will be facing our best troops under our best officer, quite possibly without the advantage of home ground.
Our second priority should be morale, both of our men and Bovianum. Securing supplies will help massively with this, but a nice string of victorious skirmishes and a sharp decrease in predation on the roads will help even more. Chasing Gemino is just playing his game and giving him oxygen, so we need a lesser calibre of opponent - the bandits. This has the additional benefit of cutting down on the number of men Gemino can potentially gather to his banner. As the Gauls are simultaneously our most restive and mobile, and least versatile troops, they are an excellent choice for this. Given their (lack of) discipline, however, I do not think that they should be trusted to operate independently of our oversight.
It is important to realise that Gemino cannot beat us conventionally without absurd luck or us conniving at our own destruction. He has to either a) break our will/ability to remain here (hence both the letter and striking at our supply lines), or b) cause a general uprising. So long as we keep Bovianum in hand and avoid any catastrophes, he is eventually going to lose, and he knows that. Sooner or later, he will have to take a chance. And that will be our opportunity, if we cannot engineer one before then.
[X] Plan Cunctator
-[X] Castrum Aestiva
-[X] Garrison
-[X] Eliminate the Bandits
--[X] Use the Gauls
--[X] Join the hunt
-[X] Establish Supply Lines
--[X] Pompolussa
-[X] Liberate the Towns
-[X] Connect With Elders
-[X] Study Logistics