Combine ground beef, 1/4 cup barbecue sauce, garlic powder, salt and pepper in a large bowl. Mix well. Shape into 6 patties. Refrigerate until grill is ready.
Heat gas grill to medium-high. Lightly coat onion slices with oil and salt and pepper. (Note: You can stick a wooden skewer through each onion slice to hold it together. Wooden skewers should be soaked in water beforehand.)
Grill onion slices over direct heat for 3 minutes per side. Then move them to one side of grill and turn heat to low on that side. Baste them with barbecue sauce and remove them from grill when they are soft and charred.
Lightly sprinkle burger patties with salt and pepper and place on the grill for 3 to 5 minutes. Flip over and continue to grill for about 3 minutes. Baste the tops with barbecue sauce. Top each burger with a slice of American cheese just before removing from grill. (Internal temperature should be 160 degrees.)
Place lettuce and tomato slices on buns, top with burgers, onions, bacon, and more barbecue sauce.