What does "ganddo" mean, specifically? I've never seen it before.
ganddo fe - with/by him. This is used in the possessive pattern - see the section notes for 'Possession' for the full pattern of gan.
I think you can use both "ganddo fo" and "gynno fo" for "he has". "Gynno fo" is the form taught in the northern "cwrs mynediad", but DuoLingo seems to prefer "ganddo fo".