Apple made the source of GCD available where it is known as libdispatch. I've used it extensively on Linux, but the port appears to be abandoned. The major release from last year never made it to the head of the repository and the mailing list is dead.
This is sad, because C+clang+blocks+libdispatch really beat the socks off of C+pthreads for very low overhead network servers.
I've never used this, but it seems to gives you the API at least, even if it might run a bit slower without kernel support. It also provides a c++ wrapper that will work with c++11 lambdas, so it doesn't even depend on clang.
