They did not copy GIMP, they literally extracted the widget code out of GIMP (and with it the object model) and made it into an independent toolkit. At the time there was no decent free toolkit at all, people were trying to reimplement Motif (Lesstif) or Qt (Harmony) but GIMP's was the first free software toolkit for X11.

Then over time it was extended beyond widgets, and the tooling around it grew to focus on exporting introspection information for language bindings.