Using self-hosted GitLab as Oauth Provider
You need to whitelist the redirect URLs of your app in GitLab.
Admin Area -> Applications -> Your Django App -> Callback URL
I managed to succefully install django-allauth and configure my app to allow authentication via Google Oauth2, but when i try to configure GitLab Oauth2 integration it juste fail with the message:

The redirect URI included is not valid.

I did set 
http://xxx.xxx.xx/accounts/gitlab/login/callback/ as Callback url 
Thanks!