The last time I've looked there were a lot of C code needed to be compiled with C to build Go, and definitely it was not for the bootstrapping purposes as the part of standard libraries were written in C. Is it still the case?
a large part of the runtime package (within the standard library) is written in c and asm. This c code compiled with one of the 5c, 6c or 8c compilers.
gcc is only used for bootstrapping (I believe... tho I am still trying to get my head around what happens).