edit to add this was in response to
@Brian G Turner 's question.
Just my impressions, from reading in the U.S. for 45+ years...gray was the spelling I was taught to use in school in the 1960s (and was the form most commonly used in contemporary American-version-of-English literature I read growing up). I would have thought, in my formative years, that 'grey' was very British (I may even have been told this by teachers), and was a more formal spelling, and was perhaps to be found most often - in books I'd have access to then, in the States - in Fantasy stories (I just checked, and of course 'grey' is used exclusively in the Tolkien Middle Earth tales, never 'gray' [at least in the three books I searched]). I just checked an old U.S. SF book, and gray was the preferred spelling.
I would have assumed that 'grey' might have been more commonly used in U.S. publications before 1900 (I searched my volume of Poe, though, and found 36 uses of gray, but also 9 uses of grey...curious as to why he'd use both; an editor's choice, or could he have been trying for a different feel in the stories that used grey?).
And the Chrons spell-check certainly favors grey over gray (which I am told is misspelled
). (It is also not very fond of 'favor', preferring 'favour'.) Well, those are the thoughts of one person who grew up in the States, on the East Coast, in the 60s and 70s.