GTK is a UI toolkit, i.e. a piece of software that draws uniform-looking buttons and scrollbars and the like.
GTK used to stand for "GIMP toolkit" but GTK and GIMP development are now entirely separate, so much so, in fact, that 13 years after the release of GTK 3 and 3 years after the release of GTK 4, GIMP still hasn't upgraded to either.