For the past few months I have been working on a simple windows notepad like text editor. It’s nothing special, but when I first switched to linux I looked around and it took me a while to find leafpad. Unlike leafpad however, Janus uses gtk3, a much more modern toolkit then gtk2, it can display and modify binary data, and it can highlight code syntax for most popular languages. Feel free to check it out on the github.
I think your use case/justification may not be applicable to the majority of GUI app developers. I have been using both professionally for at least 15 years and IMO without a doubt Qt is so much easier to use, read and work with it’s not even a comparison. When even Linus Torvalds cannot figure out Gtk or get non-rude useful help on it and jumps ship to Qt instead, I think that says it all. Plus in comparison to Qt there are almost no commercial outfits using Gtk professionally and selling products based on it.