Gilbert x Oz. Long live Ozbert! They're so sweet together. Gil does so much for Oz, and I think the bond is deeper than a master-servant bond. The only problem is that leaves out Alice (and I'm sure she'd throw a fit if her manservant was stolen by Gil), so it can be a three-way love triangle.
I'm not much into yaoi! I don't know if he will end up with anyone in the end. He might just end up being OZ's servant till he dies. Another guess is that he might just find someone one that isn't mentioned and live his life with her unless he really is too intently in love with Oz. ^_^