Trim the ends off of both zucchini. Slice into quarters. Add to a plastic food storage bag.
Mince the garlic. Add that to the bag along with the avocado oil, vinegar, sea salt, Italian seasoning, and onion powder. Marinate for 1 hour or overnight.
After the zucchini has marinated, heat up the grill to medium-high. Once hot, layer the zucchini across the grates. Check after 10-15 minutes. Once you see grill marks, flip over for another 10 minutes.