Rinse tuna and pat dry.
Combine grapefruit juice, oil and 1 teaspoon cumin in shallow glass baking dish. Add tuna; coat both sides. Let stand, covered, in refrigerator at least 15 minutes or up to 2 hours.
Combine orange, onion, cilantro, jalapeño pepper, orange juice and cumin in medium-size bowl. Cover and refrigerate.
Heat barbecue grill to medium-hot, or heat broiler. Peel, pit and chop avocado. Stir into salsa.
Remove tuna from marinade. Grill or broil, turning once, for 10 minutes or until just cooked through.
To serve, place the tuna on individual plates and top with salsa. Makes 4 sesrvings.