@CaptainJack Cross-platform? If you're a web dev already, Electron will be the shortest path (Node.js plus Chome web view bundled as a desktop app).

If you want to go native and stay cross-platform, look into C# and Mono.

Aside from that, you can create GUI software with pretty much any language on Linux, as long as they have GTK or Qt bindings.

On Windows you're more or less stuck with C++/C# as far as I know.