After you installed GitLab and on your first visit, you would’ve been redirected to a password reset screen to provide the password for the initial administrator account. You should’ve entered your desired password and gotten redirected back to the login screen.
The default account’s username is root. Provide the password you created earlier and login. After login you can change the username if you wish.
I’m thinking maybe you registered a new account instead of doing the above? Maybe try log in as the admin user with username root and password 5iveL!fe? Then edit that user as your new admin account.