GIMP has been releasing two versions for several years. First, the Stable release, which is the 2.10.x series. Second, the development release, which is the 2.99.x series, which is where the GTK3 work has been done. The work from the development release will culminate in the Stable release reaching 3.0. GIMP will continue to support 2.10.x for some time after 3.0 becomes stable, but eventually they will stop supporting it.
Most of the work right now is focused on the development release and getting GIMP 3.0 stable and ready for release, but they’re still doing a little more work to tide users over until 3.0 is out. If you’re curious how work on 3.0 is going: gitlab.gnome.org/GNOME/gimp/-/milestones/27#tab-i…
GTK3 brings Wayland support among other features and yes, it looks nicer. GTK3 is still maintained while GTK2 has been obsoleted, which means bug fixes are still landing. Once they’re at GTK3, that makes it much easier to move to GTK4, which brings even better Wayland support (i.e. color management will actually be possible) and a much better UI in my opinion.