I also use Vim and Fugitive, but instead of using a GUI I use `vimdiff` as my Git mergetool and difftool. You can use `git difftool` instead of `git diff` and y

I also use Vim and Fugitive, but instead of using a GUI I use `vimdiff` as my Git mergetool and difftool. You can use `git difftool` instead of `git diff` and your diffs will open in `vimdiff`. This also goes for any other diff programs which take command-line arguments, like some GUI programs, FWIW...