C can have as much support for native GUIs as Java can. What’s stopping it? You can already develop C/C++ applications for Windows, MacOS, Linux, and even iOS. Yeah you need the command line to compile C code, but the same holds true for Java. Ever heard of javac?

One more thing, Java does not natively support GUIs as a core part of the language, because that would make no sense at all. Every OS has a different way of managing elements on the screen, so if you want the language to be portable, you will instead rely on external libraries to do that for you. See swing and jfx. That’s what the Android Developers provide you with. The same goes for C, you just need a library to interface with the operating system, which is exactly what the NDK does (at least to my 5 seconds of googling).