I'm from the US, I use both. Mostly because I can never remember (or really care) which is the US spelling VS which is the British/Canadian spelling. They both seem equal to me. And for me, I don't feel that way about the US vs British spelling of ANY other words.
But yeah. "Grey" and "Gray" are equal to me.
I attribute my sense of equality for the spellings of both "Grey" and "Gray" to my sets of crayons when I was a little kid. I remember as a little kid, some of my crayons would be labeled "gray", and others would be labeled "grey". ...I wonder if I had Canadian crayons mixed in with my US crayons? I was close enough to the border that we went to Canada all the time, so it's possible.