Sure, gtk+ deserves a mention. A few years ago, I was studying the GUI toolkits for C and C++ that were useful for cross-platform (Linux, macOS, Windows) development, and found three contenders: Qt, gtk+, and wxWindows.
Now that I’m programming in Go, I’ve become used to the idea that things should be simple, quick, and fun. I keep wondering if it would be better to focus on a toolkit written in Go that would be cross-platform and work on every platform supported by Go.
(It’s another subject, but I want cross-platform audio support, too. Right now, Qt does that a little, but not yet really well.)