1. Preheat grill to 350° to 400° (medium-high) heat. Rub steak with 2 Tbsp. olive oil and next 3 ingredients. Brush corn with remaining 2 Tbsp. olive oil.
2. Grill steaks and corn at the same time, covered with grill lid, 7 to 8 minutes, turning steak once and turning corn every 4 to 5 minutes. Let steak stand 10 minutes.
3. Meanwhile, hold each grilled cob upright on a cutting board, and carefully cut downward, cutting kernels from cob. Discard cobs. Thinly slice steak.
4. Layer spinach, grilled corn kernels, steak, avocados, and grapefruit on serving plates. Serve with bottled Ranch dressing.