There is *no* use for this GTK-MUI library if you are not writing a program using GTK Linux libs. But if you do code. Then this version is what you should be looking for.

This GTK-MUI library is developed for AmigaOS 4, AROS, and MorphOS. The AmigaOS 4 version got uploaded to the os4depot site recently is originally done by Andrea Palmatè. Everything is provided as they are. It is now maintained by Oliver Brunner and others that have taken over the work now. It is great to see that the development of GTK-MUI continues. With this, you wrap GTK+ to Zune/MUI (Magic User Interface) wrapper library, which makes it possible, to compile GTK+ programs for AmigaOS, AROS, and MorphOS without any changes to the sources. The result is a native AmigaOS/AROS/MorphOS program with a native MUI GUI.

GTK Demonstration on AmigaOS 4 from Sourceforge

GTK-MUI is really outdated for AROS and MorphOS. You can read way more about it on the Sourceforge website. Did I found this on the os4depot site and with this attention maybe someone wants to do something about it?