Rinse fish; pat dry. In a small bowl whisk together the lemon juice, oil, and garlic. Reserve 3 tablespoons of the mixture. Brush remaining lemon juice mixture on fish and the cut sides of the squash. Lightly sprinkle fish and squash with salt and pepper. Grill fish on the lightly greased rack of an uncovered grill directly over medium heat for 8 to 12 minutes or until fish flakes easily when tested with a fork, turning once. During the last 5 to 6 minutes of grilling, grill the squash until just tender, turning once.