Place shrimp on skewers. Brush with olive oil and season with sumac, salt and pepper. Grill over medium-high heat about 2 minutes per side until pink and opaque. Remove from skewers and set aside.
Cut heads of lettuce lengthwise. Brush with olive oil and season with sumac, then grill over medium-high heat cut side down 2-3 minutes total, turning occasionally. Remove and set aside.
Grill the pita bread turning occasionally until crisp. Crumble into bite sized pieces.
Place garbanzo beans, tomato, cucumber, pepperoncini and onion in a bowl and marinate generously with your choice of dressing.
Arrange grilled lettuce on individual plates. Spoon marinated vegetables over lettuce, then top with shrimp, pita crumbles and feta.