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 firstname.lastname@example.org: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 email@example.com is working first.