Heat grill. Coat chicken with olive oil and season with garlic powder, salt, and pepper.
For the topping, heat olive oil in a skillet and saute onion and garlic until onion is translucent. Stir in tomatoes, olives and capers, if using, and season with salt and pepper. Allow to cook and thicken.
Grill chicken over high heat until cooked through, about 8 to 10 minutes per side depending on the thickness, and serve with topping.