Soak hickory chips in water to cover 30 minutes; drain. Wrap chips in heavy-duty aluminum foil; make several holes in foil. Set aside.
Brush fillets with oil; sprinkle with salt and pepper.
Light gas grill; place foil-wrapped chips directly on hot coals on one side. Coat grill rack on opposite side with cooking spray, and place on grill. Let grill preheat 10 to 15 minutes; turn off burner opposite wood chips. Arrange fillets on rack opposite wood chips, and grill, covered with lid, over medium heat (300° to 350°) 10 minutes on each side or until fish flakes with a fork.
Wrap flour tortillas in heavy-duty aluminum foil. Place on grill during last 5 minutes of cooking time to warm.
Serve immediately with Mango-Avocado Relish; garnish, if desired.