Heat gas grill on medium or charcoal grill until coals are ash white. Make 12-inch square aluminum foil grilling pan with rectangle of double thickness heavy-duty foil or use purchased foil pan. Spray pan with no stick cooking spray.
Combine garlic butter and lemon juice in medium shallow dish.
Dip each fish fillet in butter mixture; dip in instant mashed potatoes, coating both sides well.
Place fish in foil pan on grill. Grill, turning once, until fish flakes with fork (6 to 8 minutes). Serve with lemon wedges.