This is will be hard to do. As I can see the situation you have to make sockets programming and to catch the streamed data into it.
I don't if you can send the video stream via your own network protocol and to catch it on the other side.
Another simple solution is to use ready library such as libsip or something like this.
You can do it with SIP protocol, there is available open source sip library.
Have a look about this. http://www.linphone.org/
This all is not a GTK related programming but it can help you.