If you want to force the Gdiff split to vertical you coan use:
git config --global mergetool.fugitive.cmd 'vim -f -c "Gvdiff" "$MERGED"'
If you want to force the Gdiff split to vertical you coan use:
git config --global mergetool.fugitive.cmd 'vim -f -c "Gvdiff" "$MERGED"'