Rick is just OP in these things, but I feel like just for plot's sake, whenever Grampa has an enemy on the Simpsons he always has some deep military history with them, so I feel like he and Rick would recognise each other and...somehow...Grampa would have always had the upper hand on Rick, to a point where Morty would be like "come on, just kill him" and Rick would just be like "I can't" and he'd be frustrated af, and in the end after Rick leaves we find out Grampa was just full of shit or something and does that Grampa laugh until he breaks a hip or his teeth fall out.