It's theoretically possible to copy the code from Clang to GCC, but the FSF wants the copyright, so it doesn't seams to be very probable. Another problem is that the license of GCC is more restrictive, so I think that most of the people would continue to work in the Clang.

It's possible to fork the code, but it's more difficult to fork the community. It's the problem that the BDFL have to solve to survive.