Preheat grill for medium heat and lightly oil the grate.
Combine ground beef, garlic powder, and black pepper in a bowl; mix until well combined. Press into 8 equal hamburger patties 1/4-inch thick. Place 1 tablespoon olives in the center of 4 patties. Top with remaining patties; pinch edges to seal.
Grill patties until no longer pink in the center, 6 to 8 minutes per side. An instant-read thermometer inserted into the center should read at least 160 degrees F (70 degrees C). Top each with 1 slice Cheddar cheese; remove from grill.
Place each patty on a bun; top with 1 tablespoon mayonnaise, 1 tomato slice, 1 onion slice, and 1 lettuce leaf.