Using grunt gh-pages module can give the following error while pushing to the origin:

Cloning git@github.com:<organization>/<repo>.git into .grunt/grunt-gh-pages/gh-pages/src
Cleaning
Fetching origin
Warning: fatal: Couldn't find remote ref refs/heads/gh-pages
Use --force to continue.

This can occur if you have executed the gh-pages command at least once and then deleted the remote branch. Now if you again run the gh-pages command the above error will occur.

This occurs because gh-pages updates the .gitconfig under (.grunt/grunt-gh-pages/gh-pages/src/.git/config) with the reference and if you do git clean -fd it skips the .grunt/gh-pages folder.

The solution is to do rm -rf .grunt and then run the grunt gh-pages command