When I first saw your post this morning, i was going to reply that you should register an "Alarm", and then in your alarm method poll mpg123 for the current frame, and update your progress bar accordingly, but i didn't see any mention of "Alarm" in the Gtk docs (at least not the sense i ment) so i walked away from my computer for a while, did some stuff, and when i came back and saw there was still no replies, i did some digging.

The concept i was thinking of is called a "Timeout". There is a pretty good example of using timeouts with progress bar in this tutorial