GitLab Login Setting.

Login to Admin->Site Settings->Settings.

Here, you will be enable login with GitLab.

What is GitLab Social Login Callback URL?

if your domain is like that https://yourdomain.com/ then the callback URL is

https://yourdomain.com/auth/gitlab/callback

and

if your domain contents public like that https://yourdomain.com/public then the callback URL is

https://yourdomain.com/public/auth/gitlab/callback