The gtm-oauth2 controller fetches the user's email address by default, and can also fetch some additional profile information if the appropriate property is set prior to the user signing in:
viewController.signIn.shouldFetchGoogleUserProfile = YES
The profile information is detailed at
https://developers.google.com/accounts/docs/OAuth2Login#userinfocall
For any YouTube-specific profile information, see the YouTube API documentation and discussion area.