My apologies. I'd never seen it, and since it was in the context of chess, I included that in my searches. There was lots of information about cheese, and lots of questions about who the GOAT was, but not what it meant.
Interestingly, my comment got a handful of upvotes before it was modded to oblivion, so apparently I wasn't alone in not knowing this very common acronym.
I'll try to be more American next time I'm on the Internet.
