As you can see in Youtube API revision history:
The channel resource's contentDetails.googlePlusUserId property has been deprecated. Previously, the property was only present if the channel was associated with a Google+ profile. Following the deprecation, the property will no longer be included in any channel resources.
So, how can I understand wich Google account relates to a Youtube Channel?
The only option that I can think of would be to call People: get using me as the userid. This will give you the id of the current user.
People: get
Get a person's profile. If your app uses scope https://www.googleapis.com/auth/plus.login, this method is guaranteed to return ageRange and language. Try it now or see an example.
If using the userId value "me", this method requires authentication using a token that has been granted the OAuth scope https://www.googleapis.com/auth/plus.login or https://www.googleapis.com/auth/plus.me. Read more about OAuth.
GET https://www.googleapis.com/plus/v1/people/me
This will require an additional authentication scope.
If using the userId value "me", this method requires authentication using a token that has been granted the OAuth scope https://www.googleapis.com/auth/plus.login or https://www.googleapis.com/auth/plus.me
