Cut 4 (1-inch thick) half-moon slices of watermelon. Reserve any remaining watermelon for another use. Mix vinegar, oil, lemon juice, rosemary, salt and pepper in small bowl. Reserve 2 tablespoons for drizzling grilled watermelon. Place watermelon steaks in glass dish. Add remaining marinade. Refrigerate 20 minutes, turning watermelon halfway through marinating time.
Meanwhile, for the Walnut Gremolata, mix walnuts, parsley and lemon peel in small bowl. Set aside. Remove watermelon steaks from marinade. Reserve leftover marinade for brushing watermelon during grilling.
Grill watermelon steaks over high heat 2 to 4 minutes per side or until grill marks appear, brushing with leftover marinade.
To serve, cut watermelon steaks in half. Drizzle with reserved 2 tablespoons marinade. Sprinkle with Walnut Gremolata.