>Out of curiosity, I tried installing Gimp to see how much has changed (for the better?) since I last checked it out. The tool interface doesn't quite fit into its window by default so you have a tiny little scrollbar at the bottom that you have to scroll to see the full text. I resized it. After popping out a submenu, it once again didn't fit.
This is plain unacceptable. Yes, this is 99% there, it's a minor thing, it's probably thousands of man-hours to avoid a gazillion other bugs and issues and even get to this point. But it's not enough. A UI for such a tool, where you actually fiddle with it a lot, has to be perfect. Perfect on a technical level, perfect in a Apple-y "let's ensure that the empirically determined most-used features are shown first and less-used stuff doesn't clutter precious screen-space" sense that takes long and frustrating trial-and-error involving real-world feedback. You can't have a button that's 15 pixel larger than it has to be. You can't have a layout arrangement that wastes half a window because it's poorly aligned. You can't have spacing that's inconsistent enough to make you hesitate clicking an icon because you're not sure if it's connected to some other element of the interface.
Open source/free software needs a user interface push, but apparently that's not a sexy field to work on.
You really should prepare for a ton of bricks about to crush you down as people discover that you think adjustment layers, layer effects, or CMYK support is "5% of Photoshop's deep ocean of features" :)
Xamarin Studio is built using Gtk and actually looks nice on macOS, or at least I have seldom heard people complain about it:
https://blog.xamarin.com/the-making-of-xamarin-studio/
I think the problem is that Gimp is understaffed to produce a Photoshop killer with any toolkit.
Aka CADT...
1) Photoshop had a 10+ year head start and tons of patents.
2) Adobe can pay programmers to work on it as their main and sole focus. GIMP is stuffed by volunteers.
3) Adobe can pay programmers to work on boring parts. OSS programmers tend to navigate to the "glorious" and "fun" parts.
4) Adobe has extensive testing equipment and top notch relations with top designers using its products for feedback.
5) At some point GTK, the toolkit GIMP (and Gnome and tons of other stuff) is based on, had just one developer working actively on it. I know because he blogged about it, and was fed up. That's not really what you want for your core infrastructure.
6) Adobe actively maintains Windows and OS X releases. GTK has mostly given up on other platforms and had historically had lots of problems.