In a medium microwave-safe bowl, warm the grape jelly for 30–45 seconds until loosened and pourable.
Whisk the warmed jelly with BBQ sauce, apple cider vinegar, and garlic powder until completely smooth.
Lightly grease the slow cooker (optional for easier cleanup). Spread about one-third of the sauce in the bottom of the crock.
Add the frozen meatballs in an even layer, then pour the remaining sauce over the top. Toss gently to coat.
Cover and cook on High for 30 minutes, then reduce to Low and cook for 2 hours, stirring once halfway. Uncover for the last 15 minutes to let the sauce thicken slightly.
Switch to Warm and rest 5–10 minutes before serving to allow the sauce to cling to the meatballs. Serve with toothpicks or over rice.