Follow the following steps to get ready to contribute to GHC:
git remote add myFork firstname.lastname@example.org:JohnDoe/ghc.git
"myFork" can be any name you choose. The exact url is given on your fork's gitlab page.
Now you can e.g. fetch and checkout a branch from your fork:
git fetch myFork
git checkout myFork/myBranch
Or push a branch into your fork to run the CI on it and to submit pull requests:
git push myFork myBranch
Note: if you really must clone your fork directly, you can use the python script .gitlab-ci/fix-submodules.py which tweaks the git submodule paths to point to gitlab.haskell.org/ghc/ghc.