Gimp roadmap doesn’t give any dates and I asked Gimp developer Michael Natterer (aka mitch).
Mitch told us: “Next stable release will be 2.10, which will bring the gegl port, and deprecates lots of libgimp API. Then there will be gimp 3.0 which will be GTK3. There is the gtk3-port branch that it kept rebased on top of master and works pretty well. The only date i can give is that 3.0 will come after 2.10, sorry”.
So, Gimp on GTK3 won’t come any time soon, but oh well we can still try it from gtk-3 branch.