Are you saying that your Apple ID is based on your @gmail.com Account ?
The 16 character bit refers to names used with the AIM login.
AIM Logins come from Apple IDs ending in @mac.com or @me.com as well as name registered at AIM
The Apple ones need to make sure you deliberately keep to the 16 character limit (An AIM registration will do so Automatically).
(Technically you can register any active email as an AIM Screen Name in their Registation Process and I have seen both Hotmail and GoogleMail emails as AIM Screen Names - but that raises a different issue.)
A Googlemail ID used as an Apple ID and already used with iCloud should work in Messages beta.
In System Preferences > Mail, Contacts and Calendars do you have Messages Enabled against this Apple ID (Gmail ID) ?
8:26 PM Tuesday; May 1, 2012
Please, if posting Logs, do not post any Log info after the line "Binary Images for iChat"
iMac 2.5Ghz 5i 2011 (Lion 10.7.3) G4/1GhzDual MDD (Leopard 10.5.8) MacBookPro 2Gb (Snow Leopard 10.6.8) Mac OS X (10.6.8),
"Limit the Logs to the Bits above Binary Images." No, Seriously