Here's a quick and dirty port of the GC to druntime.  I've included a new posix.mak (as posix.mak.new) and the code itself in gcnew/.  What I did was rename src/gc to src/gcbase and create a symbolic link from src/gc to src/gcnew.  Then build with posix.mak.new, etc.  I marked most changes with TODO or BUG, but the pertinent bits were that I added gc_qalloc() and commented out the PointerMap stuff.  I haven't done more than verify that the GC works for some unittests so I'd love to see some more thorough testing.  Porting to Windows may be a bit more involved because I believe I saw a posix header imported somewhere that isn't inside a version(Posix) block.

http://invisibleduck.org/sean/tmp/gcnew.zip