standard .gitignore should work, why not use that?

you can also specify patterns just for git grep in your git config

but these would be global

-> https://git-scm.com/docs/git-grep

I don't use gitignore because some of the files I don't want to search are checked in

e.g. some minimized js for swagger docs

then you can call it with a universal argument and use -e option for a pattern https://git-scm.com/docs/git-grep C-u M-x counsel-git-grep