As with GitHub, you can also deposit your (Nitrokey) SSH key at GitLab.com, by navigating to Preferences -> SSH Keys. Now verify that the key is working:
$ ssh -T git@gitlab.com
Don’t forget to check that the shown fingerprint matches the one on this site. Now if your key is uploaded correctly, the Nitrokey needs to be plugged in, and you will potentially be asked to enter your Nitrokey PIN.
Change the remote of your GitLab repository to SSH:
$ git remote set-url origin git@gitlab.com:pavel-pi.de/my-repo.git
That’s it.