In Clang, this is about as much code (scattered throughout the codebase, interacting with other aspects of C and C++ language semantics in non-trivial ways, with a high testing overhead) as the entire Objective-C implementation in GCC. The complexity of adding this to GCC is greater the complexity of adding Objective-C support to GCC from scratch was.
Is this not the strongest argument to deprecate GCC, and just build on Clang/LLVM? If feature parity with Apple's API is the goal, isn't this kind of a requirement at this point?
There's other rather large projects moving purely to a llvm toolchain (android ndk), so it wouldn't be totally unheard of. They've deprecated gcc and are removing it entirely in a future release.