Stir ketchup and grape jelly together in a large saucepan or Dutch oven over medium heat; cook, stirring, until smooth, about 5 minutes. Stir garlic powder, salt, and pepper into ketchup mixture.
Stir meatballs into ketchup mixture; simmer, stirring occasionally, until meatballs are hot and sauce is slightly thickened, about 1 hour.
