Remove stems from mushrooms; reserve caps.
Combine oil and garlic in a medium bowl.
Add mushroom caps; toss lightly to coat.
Remove mushrooms with slotted spoon; place mushroom caps on piece of heavy-duty foil.
Close foil over mushrooms, sealing edges tightly.
Grill at edge of grid, over medium-hot Kingsford briquets, 10 to 15 minutes or until tender.