Though I never understood the inconsistency, I have never heard the term “Grand Aunt” used. They were always “Great Aunt”. Always understood that to mean my grandparents’ sibling, not cousin!
It’s funny, I completely overlooked the “Grand Aunt” usage. It’s always been Great Aunt and Great Uncle to me as well. I suppose “Grand” makes sense because all the “Grands” would be in a common generation and the “Greats” in the generations older than the “Grands.”
But, like you, I’ve never heard it before.