Depends. If Wallace can keep him at greatsword range, then Maximus' gladius just won't be long enough to reach Bill.
If Maximus can get inside the effective range of the greatsword however, then the short stabbing blade of the gladius is a far more effective weapon at that range.
The Scottish blades (c.30-40") are more designed for an open battle with more room around the combatants so that slashes and cuts are used more often, and blade length is more useful. The Roman gladius (c.20") is more for in the press of battle where you don't have room to swing a large sword, and stabbing from behind a large shield is a much more effective way of fighting.
So in a one-on-one situation, assuming equal skill, my money would be on William Wallace.