I have just installed GitLab.
I created a project called project-x.
I have created few users and assigned it to the project.
Now I tried to clone:
git clone email@example.com:project-x.git
It will prompted me for password
If you have the right public/private key representing a user authorized to access
project-x, then gitlab won't ask you for anything.
But that supposes that
ssh -vT firstname.lastname@example.org is working first.