Coat the grill rack with cooking spray; heat the grill.
Cut the peaches in half and remove the pits. Lightly oil the surface of the peaches. (I find the easiesat way to do this is to oil my hands and rub it on the peaches).
Place peaches cut side facing up on grill; cook until well marked on the underside, about 4-5 minutes. Turn and grill another 4-5 minutes, until peaches are well-marked but still fairly firm. Let cool.
Rub any loose skin off peaches; chop in 1/2 inch pieces and combine with the onion and olives. Season to taste with a pinch or two of sugar (if needed) and salt and pepper to taste.