GCC — or even something like Mono — doesn't come with the headers et al for the GUI parts of Windows. And those headers are protected IP, even if you re-derive

GCC — or even something like Mono — doesn't come with the headers et al for the GUI parts of Windows. And those headers are protected IP, even if you re-derive them from scratch — just ask Google about the JDK! You are arguing that a replacement for windows with an identical API would be protected IP. Not the toolchain for building applications.