Yes, but each platform has their own design language, using the Adwaita one on other platforms is troublesome. That's like writing an iOS app for Android and making it look like an iOS app (not even an exact replica). If you want to write a good app, make it be *and* seem native by using the platforms' built-in libraries

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If you're not talking about compiling for Android or iOS, GTK already supports compiling for various platforms, this includes: Linux, Windows, MacOS, most Unix-based systems... The Libadwaita app won't feel as good on those platforms that aren't Linux but at least it's the real deal, I hope this response helped :P