It really is. GTK can basically be used with any language, C bindings are extremely common and there are bindings for all my favorite languages, Go, Rust, C#, etc.

But the Windows compatibility of GTK is just horrendous. It seems to work now with the help of some people GTK 3+ can be used on Windows, but the icons sometimes don't work, transparency never works, expander window grippy thing doesn't look right, fonts look terrible, responsiveness is terrible, form fields look terrible and don't function normally, the list goes on and on.