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.