The most likely reason gcc was used was that Ian Taylor, the guy who is part of Google's core team and wrote gccgo, was already familiar with gcc by virtue of being long-time gcc developer.
>Does this mean compilation speed, performance speed, or both? Or is this a hand-wavy excuse to use gcc instead due to Ian's expertise?
IIRC that remark related to using LLVM or GCC as the backends for the Go compiler, basically they found both backends to be too large and slow for their liking so they wrote their own full compiler from scratch.
I believe the GccGo project is mainly about leveraging all the architectures which GCC supports, many of which Go isn't ported to and possibly won't be ported to (as in maintained by the Go developers).
There already is a project to build a go compiler on top of llvm (https://github.com/axw/llgo).