Combine yogurt, cumin, paprika, and garlic in a bowl. Cut lamb into 24 (1-inch) cubes; sprinkle with 1/2 teaspoon kosher salt and 1/4 teaspoon black pepper. Combine lamb and 1/2 cup yogurt mixture in a zip-top plastic bag (refrigerate the remaining yogurt mixture); seal and marinate in refrigerator at least 1 hour. Remove lamb, and discard marinade. Thread 3 lamb pieces onto each of 8 (10-inch) skewers. Heat a grill pan over medium-high heat; coat with cooking spray. Arrange 4 skewers on pan; cook for 7 minutes or until desired degree of doneness, turning frequently. Remove from the pan. Repeat with remaining skewers. Serve with remaining yogurt mixture.