I understand that you don't support VoiceAndVideo yet, but what about chat? Can I chat over the Gizmo Project in Adium?

Yes. Gizmo's chat system is simply Jabber, so all you need to do is add the account as a Jabber account.

Your Jabber ID is username@chat.gizmoproject.com. For example, if your Gizmo Project username is jdoe, your Jabber ID is jdoe@chat.gizmoproject.com.

Once this account is added, you can chat with anybody using any Jabber-compatible service, including Jabber, GTalk, LiveJournal, and Gizmo (see ServiceInteroperability).

Isn't there a Gizmo Project plug-in for Adium?

Yes (it's listed on the Adium Xtras site), but it doesn't work with 1.0. They are waiting for us to complete a stable plug-in API, and then they will update the plug-in. (Thanks to Adium Xtras user bikefridaywalter for linking that forum topic on the Xtra page.)