It's not about the GPL. Historically, RMS has opposed any effort to make intermediate outputs of GCC available (e.g. the AST, type information, or even having a stable plugin API). On the basis that it would then be used to build closed-source products that use GCC as a service, rather than those products contributing to GCC. It might have been prompted by earlier efforts by Apple to make an Objective-C compiler built on GCC that isn't GPL-licensed, which obviously ran foul of the GPL. Ironically, that's why Apple invested in clang and why today we have the rich clang/llvm ecosystem.