Blend yogurt, cucumber, green onions, mint, and sugar. Cover and chill to let flavors blend. Combine vinegar, oregano, garlic powder, salt, pepper, onion powder and cumin.
Cut frozen steaks into 1-inch strips and cook as directed on package, stir in vinegar and herb combination and toss to combine.
Assemble each gyro with steak mixture and fresh spinach. Top with yogurt and cucumber sauce.
Makes 6 gyros