GTK, Qt, WxWidgets, FLTK are more or less your only choices for generic, long lived, cross-platform desktop GUI toolkits. I apologize for any that I've forgotten, except Tcl/Tk which is not seriously an option at this point in time :)
There are some "promising new up and comers", including libUI.
If you work in the audio software domain, JUCE is likely your go-to cross-platform toolkit.
I settled for Yue: https://github.com/yue/yue It's been around for several years, though definitely doesn't have the developer or user base of other well-known frameworks. The deciding factor for me was that is has well maintained Lua bindings as part of the core project, alongside JavaScript (Node.js) and C++.
I didn't have much luck with libui (crashes, missing features, etc), and various immediate mode alternatives just require too many dependencies and other work that made integration painful. Plus, Lua bindings for these were always stale. In fact, Lua binding quality is pretty poor all around, including GTK, Qt, WxWidgets, and FLTK.
There are some "promising new up and comers", including libUI.
If you work in the audio software domain, JUCE is likely your go-to cross-platform toolkit.