fzf.vim is great, but its :GFiles always assumes that .git is in a particular place. It can be made more robust to this and other edge cases by integrating with Fugitive, which already abstracts the details of interacting with Git.

FugitiveFiles.vim

" Alternative to GFiles: integrate with Fugitive to work better in edge cases
" Completion should actually be pathspec, not file, but close enough
command -bang -nargs=* -complete=file FugitiveFiles
\	call fzf#run(fzf#wrap('gfiles', fzf#vim#with_preview({
\		'source': FugitiveShellCommand('ls-files', '-z', '--', <f-args>),
\		'options': '--read0 --multi --no-clear --prompt=' . shellescape('FugitiveFiles> ')
\	}), <bang>0))