GIMP 3.0 has been more than one decade in the making as the port from GTK2 to GTK3, also transitioning away from Python 2 to Python 3 support, and a wealth of other improvements from the UI to lower down into enhancing this open-source Photoshop alternative.
The GIMP project announced on X/Twitter today that they have entered the string freeze for this much anticipated release.
GTK 2 has been EoL since 2020 (GTK 3 released in 2011). GIMP 3 marks the completion of the GTK 3 port, which by itself offers:
Moving to an actively supported version of GTK (and future migrations will be easier because the difference between 3 and 4 is a lot less than the difference between 2 and 3)
Better graphics tablet support
Better handling of HiPPI displays
Better Wayland support
Should also mean that they finished refactoring the code, thus making it easier to implement new features.
And on that last point, I would say that the biggest benefit overall with the release of GIMP 3 is that we'll finally, finally start seeing serious work on implementing non-destructive editing; I've read that some of the preliminary work is going to be shipping with the 3.0 release.
The biggest thing is probably non-destructive editing, so you can do stuff like apply filters without them changing the underlying image. Gtk3 should add better support for tablets and wayland. There's also better layer tools and font support. A lot of it was on the backend, which should eventually allow for using other color spaces like cmyk natively.