Combine first 3 ingredients; set aside.
Cut cake into 8 slices. Coat grill rack with cooking spray; place on grill over medium-hot coals (350° to 400°). Place cake slices on rack; grill, covered, 1 minute on each side or until toasted. Place 1 cake slice on each of 8 individual serving plates. Top cake slices evenly with peach mixture and whipped topping.