How can I do the same instead of origin using a path/to/github/repo.git ?
git diff https://github.com/malarres/universal.git/GPII-795..HEAD
That's not the way git diff works. If you want to diff between your local repo and another repo, you need to do the following:
- Add the latter as a remote of the former. Note that nothing prevents you from defining multiple remotes, in addition to - origin, within one repository. (You may want to choose a less generic remote name than "other", though.)
 -  git remote add other https://github.com/malarres/universal.git/GPII-795
 
- Fetch everything from that remote: -  git fetch other
 
- Run the appropriate - git diffcommand, for instance,
 -  git diff other/master..HEAD
 
If you later want to remove that remote from your repository, you can run
git remote rm other