Preheat the grill or warm a grill pan for medium-low heat. Place the peaches face-down on the grill or grill pan for 8-10 minutes. When they are tender, they are done.
Notes
While you can certainly brush the peaches with a bit of olive oil or melted butter before grilling, I've never found that to be a requirement. If you want to make the peaches a bit sweeter, melted butter with a sprinkling of brown sugar is delicious. For a savory twist, try brushing them with olive oil and sprinkling them lightly with minced, fresh herbs like basil, rosemary, or thyme.