Google has just announced that they are replacing video chat in Gmail with Hangouts.
Hangouts is already a core feature of Google+ and the same technology would power video calling inside Gmail.
Hangouts would allow Gmail users to chat with as many as nine friends at a time provided all of them have active Google+ accounts.
Google added about the compatibility issues:
You’ll be able to chat with all the same people you did before and, in fact, with Hangouts you’ll now be able to reach them not only when they are using Gmail but also if they are on Google+ in the browser or on their Android or iOS devices.
All Gmail users will benefit from this upgrade, but if you and the person you’re chatting with also use Google+, you’ll get even more from the Hangouts experience. You’ll be able to video chat with up to nine people at once, watch YouTube videos together, collaborate on Google documents and share your screen.