There are two Go compilers gc and gccgo. gc is based on the Plan 9 C compilers and has very little optimisation; gccgo is a front-end to gcc and as such has all of its optimisation. gcc's concurrency support is thread-based so slower than gc's which is coroutine based.

tl;dr:

gc: slower code, faster concurrency.

gccgo: faster code, slower concurrency.