What is Gitlab's instance domain?

If you're using Gitlab on gitlab.com then the domain is gitlab.com, so you should run  ssh -T [email protected]

Gitlab can be installed locally so Gitlab instance is what is being run locally. E.g., if one runs Gitlab instance at gitlab.example.net the domain is gitlab.example.net, and the ssh command is ssh -T [email protected].

For Gitlab at gitlab.com the command is ssh -T [email protected].

This is not a real definition but one section below in that same page you can find the example of the content of a file ~/.ssh/config

# GitLab.com
    Host gitlab.com
    Preferredauthentications publickey
    IdentityFile ~/.ssh/gitlab_com_rsa

# Private GitLab instance
    Host gitlab.company.com
    Preferredauthentications publickey
    IdentityFile ~/.ssh/example_com_rsa

which links the notion of 'instance' to the notions of 'private' and 'company'. Not a foolproof hint, but at least it introduces something clearly distinct from GitLab.com proper.

Basically, if you are a vanilla user, it will be just fine to type ssh -T [email protected].

That instruction is a test before starting the dance. Worked for me also down the line, when I cloned the repository I was interested in with a git clone [email protected]:<myreponame>.git.