1. Preheat grill to 350° to 400° (medium-high) heat. Remove and discard root end and dark green top of leek. Cut in half lengthwise, and rinse thoroughly under cold running water to remove grit and sand. Cut into thin slices.
2. Toss together leek, potatoes, 1 Tbsp. garlic seasoning, 3 Tbsp. olive oil, and 1 tsp. salt.
3. Divide leek mixture among 2 large pieces of heavy-duty aluminum foil. Bring foil sides up over mixture; double fold top and sides to seal, making packets.
4. Grill foil packets, covered with grill lid, 12 minutes.
5. Meanwhile, toss together chicken, 1 Tbsp. lemon juice, and remaining 1 Tbsp. garlic seasoning, 1 Tbsp. olive oil, and 1/2 tsp. salt.
6. Shake foil packets, using tongs, and return to grill. At the same time, grill chicken, covered with grill lid, 5 minutes; turn chicken. Place green onions on grill, and grill chicken, onions, and foil packets 4 to 5 minutes or until chicken is done.
7. Open foil packets carefully, using tongs. Arrange grilled vegetables, chicken, and green onions on a serving plate. Drizzle with remaining 1 Tbsp. lemon juice.