Cut potatoes diagonally into 1/4-inch-thick slices. Sprinkle with salt and pepper; place in a heavy-duty zip-top plastic bag. Add 3/4 cup dressing, shaking to coat.
Coat food rack with vegetable cooking spray; place on grill over medium-high heat (350° to 400°). Drain potato; place on rack. Grill, covered with grill lid, 20 minutes or until tender, turning occasionally and basting with remaining dressing.