Do you have any suggestion about playing videos using GTK+?
For Playing Videos on GTK+ and other GTK Bindings you have a lot of options.
Option: Use A Third-Party Library
1- Try using ogmrip-gtk , A set of Gtk Interface, which allows you to use the open-source OGMRip library as a Gtk-Widget.
2- You may use another library, gstreamer. Also can be used easily with Gtk.
3- You may use LibVLC - gtk. A GTK wrapper for LibVLC (ever used the VLC Media Player?). Personally , I like this a lot.
Option: Using Code from Open-Source Software
1- The Banshee media player is open-source and although it uses Gtk#, you may have no trouble converting the code to GTK+.
2- See the MPlayer or Totem Player Source Code. (or any other for that matter, here is a list )
Option: Use A Process
1- I've heard the MPlayer Command Line is pretty simple. Here's a guide. http://www.mplayerhq.hu/DOCS/HTML/en/commandline.html
With GTK+ there are always a lot of new and innovative (not to mention open-source) ways to do stuff. So always keep looking for better ways. I am personally a great fan of the GTK+ toolkit and have found that there is nothing you cannot do with it.