That's really interesting, given that GIMP was originally the "G" in GTK. I would have naively expected GIMP to be a GTK 3 early adopter.
Re:Gimp 2.10.12
by prokoudine on Thursday June 13, 2019 @11:57AM
(#20176106)
Technically, Inkscape is another project, though 1.0 alpha does use GTK3.
There is no estimated date, but we hope to release 2.99.2 later this year. There are still some color management related issues to fix before we can do that.
https://www.gimp.org/donating/