I've managed to stay quiet throughout this discussion because I wanted to hear some of the arguments and counter-arguments before formulating my own opinion. As of this moment, I'm still not convinced that dropping GCC is a good choice.
I believe dealing with an all-or-nothing option is not the right attitude. At the end of the day, completely dropping GCC is going to cause major disruption to the project. The fact is that some developers will be left out in the cold if/when this happens. Another argument is, for all it's flaws, GCC is the standard compiler on most Linux distributions (any of the ones using glibc, since it does not build with clang), so it is something that we get "for free".
How about a compromise? How hard would it be for GCC to drop the GCC runtime in favor of GNUstep runtime, and implement everything except ARC and blocks (these seem to be the biggest road blocks)? While ARC and blocks are important for many developers, GNUstep itself is not directly dependent on the compiler supporting these features. Ideally, we could get to a situation where GCC and Clang compiled binaries can interoperate. Or am I way off?
One more questions... what do the GCC Objective-C maintainers have to say about this discussion? It would seem that GNUstep is now their only downstream "customer". Are they open to working with us to provide a more compatible compiler?
At the end of the day, GNUstep has been around for a very long time, and like it or not, backward-compatibility is important. I personally believe, based on some of the discussion here, completely dropping GCC is going to be cause more problems than it solves. Whatever the decision, the implementation should be well planned and deliberate.