Gmail chat is a relatively new instant messaging client which is compatible with other IM networks. At present it is only available to users of GMAIL and Google talk.
The Google Talk applications implement XMPP (the Jabber protocol). Clients which can use this protocol are widely available, so the lack of official Google clients on non-Windows systems is not an issue.
Using Jabber and associated technologies it is possible to talk to users of Google Talk even if you have a different Jabber client and do not have an account with Google.