I had a strange problem with categories today & thought I would share my fix.
For some reason I couldn't see a category or its sub categories in gravity form. Even after deleting them and recreating them i still couldn't see them (category was called Greater incase you wondered) I noticed I had another parent category called Advert not used by any subs, sitting above the other parent called uncategorised which was being used for all subs. I deleted Advert and then I was able to see the invisible categories within the gravity form. Maybe i did something wrong, but if you have the same problem, maybe try and delete unused parents.