The morning before an afternoon barbecue, make marinade by combining oil, lemon juice, garlic cloves, herbs and pepper in a medium bowl. Remove about quarter cup to mix with the onions. Place lamb cubes in bowl with remaining marinade; toss to coat well.
Place onions in small bowl and toss with reserved marinade. Cover with plastic wrap and chill, up to several hours, turning several times.
Meanwhile, make yogurt sauce by lining a medium bowl with cheesecloth and spooning in the yogurt. Tie up edges of cheesecloth with string to form a pouch. Suspend over bowl (tying from refrigerator rack above bowl), or place in a colander over bowl and let stand to drain several hours. Moisture will drip from the yogurt, giving it a thicker consistency. Mix drained yogurt with minced garlic, chopped herbs, lemon juice, oil, salt and pepper. Cover and chill. Makes about 1 cup.
Stack pita bread rounds into 2 piles and wrap each in foil, dull side out.
To barbecue, heat coals or gas barbecue. Remove meat from marinade, reserving some marinade for basting. (Discard any large garlic pieces.) Thread lamb onto greased skewers and grill over greased grill about 10 cm from hot coals. Cook, turning and basting often, about 10 minutes or until lamb is brown on the outside, still pink inside. Warm pita rounds over barbecue.
Before serving, lift the onions from the marinade with a slotted spoon and place in a serving bowl. Cut whole tomatoes into thin wedges and place on plate. Place lettuce leaves on plate. Set out yogurt sauce and lots of napkins to facilitate handling of gyros.
To serve, cut each warmed pita bread in half and separate to form 2 pockets. Each person fills a warmed pita picket with some of the grilled lamb cubes, lettuce, tomatoes and onions, topping with a spoonful of sauce.