See the online documentaion: Single Machine | Multi-Machine
Go to your GitLab profile setting and navigate to your account settings and choose Applications from the menu.
Please make sure that your drone Redirect URI setting. It is very import the authorization callback URL matches your http(s) scheme and hostname exactly with /authorize (0.8) /login (1.0) as the path.
Copy Application ID and Secret
Open your drone home page in your browser and login as gitlab account.
