I have to solve some conflict after a git pull.
git pull
$ git pull CONFLICT (rename/add): Renamed vignette_generator_mashed.h->vision_problem_8.h in
I think you just forgot "-t" switch at your command line. According git help page it stands for "-t , --tool=" so it makes what you intended to.
Try:
git mergetool -t gvimdiff
Of course you may use your prefered merge tool instead of mine gvimdiff, meld is great too...