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: