This seems odd to me - "games of football" looks like a partitive genitive ("darn o bapur", "peint o gwrw"), rather than a possessive genitive (NOUN + NOUN).
Right. But I would translate that into English as "football games".
But Duolingo gave me the English sentence "I like games of football." Genitive, not adjectival. I believe I translated it as "Dw i'n licio gemau o bêl-droed," and got dinged for it, but at this point I can't remember for sure.