Well, GIMP uses GTK and it looks really sharp on all platforms. It all comes down to theming.

I wouldn't say it looks like garbage, but if you want a more native look and feel you could certainly set it up in the settings.

(For example: you could look at https://stackoverflow.com/questions/37035936/how-to-get-nati... )

As a library purely coded in C, I still believe that GTK and GNOME lay a solid foundation for easy development in C.

Before downvoting you might want to give GTK another chance. Even this libui library uses GTK under the hood for linux.

It looks like garbage on anything not GNOME 3 with default settings.