You control a creature that has a +1/+1 counter on it.
Steps
1Activate a creature using Grist's first loyalty ability by putting a loyalty counter on it, causing you to create a 1/1 Insect creature token and mill a card, and then put a loyalty counter on the creature and repeat the process if the card was an Insect.
2The Insect enters the battlefield with a +1/+1 counter on it due to Metallic Mimic, causing it to have Grist's abilities.
3Repeat from step 1 using the Insect token instead.