When the Persist Creature dies, its persist ability triggers, returning it from your graveyard to the battlefield with a -1/-1 counter on it and at least one +1/+1 counter on it due to
Grumgully.
Remove a -1/-1 counter and a +1/+1 counter from that creature as a state-based action.
Resolve the
Grimgrin ability.
Repeat.
Results
Infinite +1/+1 counters on a creature.
Infinite creature ETB.
Infinite creature LTB.
Infinite creature sacrifice triggers.
Infinite death triggers.
Metadata
In 2 decks according to EDHREC.
Bracket Tag
This combo has been estimated to be Oddball (Bracket 2-3+).