I'm not sure how the GPL effects anything here. This is a local service, not an internet service. And the GPL doesn't cover the output of GCC at all. This also doesn't actually produce the end binaries.
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.
