Ghans is right: It's pure Python code and uses TKinter for the GUI. For a gstreamer-1.0 application you would have to use GTK+3 and gobject, which are much more complex to use (especially gstreamer os a real pain). omxplayerGUI can also be imported as a module into your own Python application.
Minimal Kiosk Browser (kweb)
Slim, fast webkit browser with support for audio+video+playlists+youtube+pdf+download
Optional fullscreen kiosk mode and command interface for embedded applications
Includes omxplayerGUI, an X front end for omxplayer