I am not an expert for DnD lore.
Are the great old ones the same as the
Qlippoth, creatures that lived in the abbys before the demons ruled it?
This is a link from pathfinder rules, but the stuff about monsters seems to be the same as DnD, only the gods are different.
I fought Demogorgon in BG2. It was a demon lord, and when it is a demon it cannot be a great old one, I guess.