Google Chat used to be able to be used within Gmail, Orkut and iGoogle, but the last two services were shutted down, so, at this time, Google Chat could be used only within Gmail.

It's worth to say that at this time there are two alternatives for chatting within the Gmail web interface, the "old chat" (Google Chat) and Hangouts. Each of them have their own help centers and and have different user interfaces and some distinctive features, like Hangouts in Chrome doesn't require a plug-in for audio and video conversations, among other features.

The gmail-chat doesn't have information in its wiki tag page. Previously, the google-chat and google-talk were merged, as I understand this, because in the context of Web Applications the users refers to the same service.

References
Should “google-talk” and “google-chat” tags be merged?
Chat Help - Google Help
Hangouts Help - Google Help