It seems that there is an error in the documentation or api jar.
In the documentation the full scope is:
In constant api, GmailScopes.MAIL_GOOGLE_COM the value is:
When there is a difference because of the "\", the system returns:
{
"error": "access_denied"
"error_description": "Requested client not authorized."
}
To have no errors you must use the same scope. With or without \ on both side.
Ref: