Here is the google help page on how to find the current Google IPs. Shaping this way though, as mentioned above, will probably cause issues, because these IPs also are for google.com, gmail.com, etc.
Like i said,I suspect there is no absolute ip cidr for a specific service but you can furher distinguish a service (like mail.google.com) by defining an alias for it too and creating a new rule before the "google" rule.
As for today gmail uses 173.194.0.0/16 if anyone interested in checking that in following weeks or months.