The regex works, though may not be optimal. I suspect mail.google.com is whitelisted above the regex trying to catch it, therefore the whitelist rule is taking precedence. To combine this with blocking other webmail, the rules need to be in this order:
Blocking regex (see below for tweaked version)
Allow rule for mail.google.com
Block rule for all other webmail.
@ Sheridan, I've tested this on the same version you're running. It doesn't matter whats above, below or between them as long as they're in that order.
(gausr).*%(40|2540)(?!your\.domain\.here)
(Don't forget to escape your dots in the domain name)
Hope this helps.