In my opinion, the question should not be about which language "should be used for GUI development", but what are your requirements? Do you need performance? Then go for a performance-language, like C, C++, Rust. Do you need something highly standardized many people are familiar with? Then go for HTML+CSS+JS. Do you need something in between? Python will be a good friend! There are many more, and just like @luord wrote, you can use any language which has bindings to graphics libraries, like Vulkan, OpenGL, Direct2D/Direct3D, GDI,...
For each of the platforms, there are many many many libraries and frameworks which make your job easier, so you might want to take a look and decide for one of them. A few of the most well known include Qt, GTK+, wxWidgets, .NET graphical components,...