I think there was a port of gcc at one time. It should be possible to use that to port later versions of gcc.
Go is already ported, AFAIK, but I have not yet found an excuse to try it out.
Personally, I would really like to have Ada (Gnat) working on Plan9. I have made enough errors in C and C++ for a lifetime...
If you have gcc, it does not take that much work to get Gnat working for a new platform. Most of the work would be the interface to the tasking system, I think.
Mercurial (and Python) already runs on 9Front.