Mountain Lion Messages: Gmail user is not registered with iMessage I'm trying to use Messages instead of Adium, but it won't let me chat with some of my contacts.

These aren't iMessage accounts, but gmail users. If I try to message them, I get a "address is not registered with iMessage." alert.  There doesn't seem to be any reason or method to it, either.  I've tried restarting iMessage as well as rebooting my machine.
Add the Google account to the Messages app. After that, you can use Messages with GTalk.