GTK is shorthand for something that started out as the GIMP Took Kit, but now that library is used by a lot of other software, much of it GNOME-related. This is something that already we explain above in May, when we were talking about how almost everyone is moving up to GTK4 and GIMP is still on v2 of "its own" library. In any case, they have been working on getting up to GTK3 for some time, and a few hours ago they released a GIMP 2.99.16 in which that part has already been terminated.
GIMP 2.99.x is the name and numbering of the preview versions of GIMP 3.0, which in turn is known internally as the GTK+3 port version. Therefore, the fact that the task of uploading it to GTK3 has already been completed makes us think that the release of the stable version is close. It's not something I say; is something Jehan says in a post on the project's official blog, more specifically “We are closer than ever to releasing GIMP 3.0, we present the latest development version: GIMP 2.99.16!”.
GIMP 2.99.16 brings us closer to GIMP 3
Now, that does not mean that it will come out in the next few days. In fact it is not known when it will arrive GIMP 3.0, and all they have said about it is that "This is because we plan to release GIMP soon enough to ensure that it can be packaged in all reasonably recent distributions..”. It is known that there are still two important things to do, one of them being the redesign of the API and the other the project of the space invasion ("Space Invasion").
For everything else, GIMP 2.99.16 has come with many new features, such as improvements in the text tools, aligned and distribution and the unified transformation tool and also in the file formats. Anyone who wants to try it, and does not use macOS, can download it from this link.