Girp is a mash-up of the words "git" and "grep". It is a Vim plugin that is a wrapper around git grep. Girp allows you to easily run git grep from within Vim and it populates the quickfix list for you.
Using Vundle:
Plugin 'aaronraff/girp'
Just use it like you would git grep! It just calls git grep -n ... under the hood.
:Girp My-Search-Term
:Girp -i my-search-term
You can also use :GirpCurrentWord to search for the word under the cursor. It can be handy to create a mapping for this in normal mode.
nnoremap <leader>g :GirpCurrentWord<CR>