Cut the zucchini and yellow squash into long quarters and place in a long container that has a lid.
Mince the garlic.
In a small bowl, combine the garlic, avocado oil, vinegar, lemon juice, Italian seasoning, thyme, salt, and pepper. Pour this over the zucchini and yellow squash. Put into the fridge for 2 hours or more to marinate.
Heat up the grill to medium high. Place the marinated squash directly on the grates, crossing the grates perpendicularly.
Grill for 20 minutes, undisturbed.
Flip squash over and grill for another 10 minutes
Remove the squash from the grill and serve. Pour any remaining marinade on top of the cooked squash.