A "native" version of GTK had been in the works for years, mainly supported by people who wanted to run other programs with it (CinePaint, I think). It seems that nobody had the interest in getting Gimp working with it, the skills to accomplish that, the time to dedicate to it, and the spirit of sacrifice needed to work on a GTK 2 port when GTK 3 is already out. But somehow, in the end, the stars aligned.