Work with vim-fugitive
So are you saying that I can use this with vim-fugitive? I have tried setting let g:committia_open_only_vim_starting = 1 but when I do :Gcommit it brings up the default committing window

So are you saying that I can use this with vim-fugitive?
No. vim-fugitive opens commit buffer with uncommon way. So currently commitia.vim can't handle it correctly.
When let g:committia_open_only_vim_starting = 1, committia.vim do nothing on :Gcommit. Then you can avoid the collision between committia.vim and vim-fugitive.
Just a note, I think the same thing happens with neogit.
This works, https://github.com/yuki-yano/dotfiles/blob/0a1bb68e0e9bc5332d366fcf5aec71d1c1468598/.vim/lua/plugins/git.lua#L80
Need to manually open it.