I think GTK is an excellent library for C. If you use Glade and GTKBuilder, you don't have to write the boilerplate either. You design you UI in Glade visually, add your signals, set your id's etc. Then you save your design as an XML file. In your C code you load that design with a simple function, then assign the function names for the signals you've created in the visual designer. Then you're all set to go.
The library (GTK) feels the same in every language, whether it's C, Python, Go, etc.
I admit, it can be a hassle to get the library up and running, but once you do that, it is surprisingly simple to code in.
The problem is it's not well integrated on anything else than Gnome. If you are on KDE, Windows or Mac, stuff like the file picker will make the experience painful.
Even electron apps don't try to reinvent this one and pop the OS widget.
The library (GTK) feels the same in every language, whether it's C, Python, Go, etc.
I admit, it can be a hassle to get the library up and running, but once you do that, it is surprisingly simple to code in.