How active is your board? Would it be overkill to set registrations to admin validation for a while, at least until the current spam wave dies down? What if you put new members into a different group that you have set to moderate posting for the first few posts and promotes them to the regular members group afterward? Granted, it would affect all registrations, not just GMail, so it could be overkill depending on your needs, but it is one way of dealing with it.