Gretel Doofenshmirtz was Heinz Doofenshmirtz's great-grandmother who was a great cook, specifically in creating a meatloaf recipe with the secret ingredient of "Hate". She passed it onto her son: José Doofenshmirtz who passed it on to Heinz Doofenshmirtz's mother who passed it onto Roger Doofenshmirtz. Heinz then stole it ("Meatloaf Surprise"). According to Heinz Doofenshmirtz, Great-Grandma Gretel had "Issues". Heinz might've known his great-grandmother personally since he said that she had "Issues" as if he knew what they were. Of course, it is possible that he was told this by his Grandpa José or his parents.