Gitlab documentation, years later, is still not clear on this. In order to mirror by pushing from your gitlab to github using ssh keys, you need to use the format: ssh://[email protected]/organization/yourproject.git

Click the button to load host keys.

Then, in the username, you should also put "git". You won't be entering your login username anywhere.

Then, you will save and gitlab will give you the option to copy the public key (it has quietly generated a private key for you and stored it). Take that public key and go over to your github repo that is the target of the mirror, Settings > Deploy Keys and add a key. Give it a name "gitlab-mirror" or whatever, and paste the public key from gitlab into the field and remember to check the "write access" option and save.

Then you can go back to your gitlab mirror page and click the "update now" button to test it.