Can you tell us what you thought it might have been?
I knew it was the address of the gitlab server, but since the docs’ example gitlab.example.com uses a subdomain, I thought I might have to use one too, like my username or something. As an inexperienced user, I was configuring my account without being conscious that you could host your own gitlab instance.
The doc is clear, but maybe specifying that the instance url for gitlab’s instance is simply gitlab.com would be clearer for new users.
In that case I can see why the text is slightly confusing. I have created
to have the page improved a bit.
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Thanks! 
The instance URL of any GitLab install is basically the link to the GitLab you’re trying to connect to. For example, if your project is hosted on gitlab.example.com/yourname/yourproject then for the instance URL enter https://gitlab.example.com .
just do this “ssh -T git@gitlab.com” and it works.
Type yes and you will get a welcome msg.
This worked!