i don't think it's possible to video chat in gmail but if you want you can download google talk. it's a pretty small application and only takes a few minutes to download and install and you can use it for chat, calling, and video chatting with google talk, except from a separate application instead of inside gmail. it's also good because when you get a new email it gives you a little notification in the bottom right corner of your screen so you know when you have unread emails. other that downloading google talk i'm not aware of any other way. hope that works!
actually when i went to get the link, it turns out that there is a voice/video plugin for gmail which might also work. i haven't had experience with it but you can try either one out. the only thing is is that google talk application is only for windows, but the voice/video plug-in is for mac too. you can read about both of them in the source.