Most parts of Go are licensed under a three-clause BSD style license and patent grant. This includes the gc compilers, standard libraries, and other related tools. There are two notable exceptions I'm aware of:
The language specification and other website documentation are licensed under the Creative Commons Attribution 3.0 License. Scroll to the very bottom of any page on golang.org to see. (I suppose this means that some source code comments are licensed the same way, which seems a little strange.)
gccgo's frontend is uses the same BSD style license as the rest of Go, however because it must be linked with the rest of gcc, it's effectively GPLv3. (Note that, contrary to janneb's claims, gccgo does not share any code with the other compiler. It does share some runtime code.)
There are ongoing efforts to separate out the parts of the gccgo frontend that depend on gcc so that it could be used as a frontend for other Go compilers, such as maybe an LLVM-based compiler. Such a compiler could be released completely under the BSD style license.
