these features are available in Core, all Enterprise features are marked as such in the docs. This is a marker on the right hand side of the chapter.
Using GitLab as OAuth provider will require administrative access to the GitLab instance, which you only get on a self-hosted installation. GitLab.com is operated by our infrastructure team, no-one else has administrative permissions.
To my knowledge, KDE, Gnome and Debian have implemented the OAuth provider as SSO on their self-hosted instances, in case you are looking for inspiration how to do it, next to the docs