In my view the primary reason for GTK’s prominence on Linux rides on one thing primarily: it’s got C bindings, and thus it has decent bindings for just about every other language under the sun too. If you’re not trying for anything fancy, these bindings can even be auto-generated.
The runner up Qt is much more tightly tied to C++ and Python to its detriment. You really need to meet devs where they’re at instead of insisting that they adopt a particular language to use your UI toolkit.
Aside from that, at the end of the day, if you’re building a full fat complex desktop app an old style imperative UI toolkit is probably one of the more practical choices you can make. They have an exhaustive set of battle tested, accessible widgets built in and their pitfalls are well known. Newer approaches expect you to write or import everything and start requiring increased contortions from the developer past a certain point of complexity.