If by "the free software world" you mean "the GNU/GPL zealot world", then please say so. Not only is GCD free software, but lots of other free software, including LLVM, is compatible with it. And lots of people are interested in LLVM and working with it.
If the majority of free software licences are GPL-incompatible, the problem is with the GPL, not with all the other licences.
As for "only runs on MacOS:" I predict that won't be true very long. And I believe that's why Apple released the source.