In a small saucepan, bring the grapefruit juice, vinegar, syrup and garlic to a boil. Reduce heat; simmer, uncovered, for 5 minutes. Transfer 2 tablespoons to a small bowl; add oil. Set remaining glaze aside.
Sprinkle salmon with salt and pepper; place skin side down on an oiled grill rack. Grill, covered, over medium heat or broil 4-6 in. from the heat until fish flakes easily with a fork, 10-12 minutes, basting occasionally with maple-oil mixture. Drizzle with reserved glaze. If desired, garnish with fresh thyme sprigs.
