A GT isn't a match for an equal sized mbuna; IME the rep of GTs bein' aggressive fish is overplayed and they are a lot more docile than many, many species of cichlids. I don't know of any geophagine cichlid that is aggressive enough to hang with Africans at any size.
If ya really want to mix mbuna and New World cichlids, which I don't recommend BTW, yer best bet is gonna be to stick with the common, tougher mid-sized CA cichlids. I know people who've mixed convicts, firemouths, salvini and dempseys with mbuna with some success. Anything much larger than a dempsey (RDs, midas, jags, etc.) will destroy mbuna once it outweighs them. If ya insist on mixin' SA cichlids with Africans, stick with similar-sized haps. Again, I don't recommend it, but I have seen some successful set-ups of haps with SA fish.
Good luck.
WYite