Pretty much. The GIMP developers couldn't find a toolkit that fit their needs so they wrote their own. Other people with the same problem then used their toolkit.

Back when the GIMP was developed your options were pretty limited. There was Athena Widgets which are basically the cave drawings of the computer world. Motif which had licensing issues. OpenWindows widgets which were notoriously buggy and ugly. There wasn't much else to choose from. There's a reason GTK exploded so quickly after release.