I don't know if this has been mentioned on here before, but has anyone ever considered that perhaps Cid decided to use GF specifically to make them forget. That way they could fight the sorceress without having to go through he pain of knowing that she raised them, and the resulting hesitation. I mean he did go out of his way to get them all back together to fight her, so this seems likely. With that said, the whole "I knew you guys all along, but I forgot" thing doesn't seem that far-fetched to me.