Heat 3 or 4 burners of a gas grill on high for 10 minutes. Sear tri tip on fat side first with lid down for 2 or 3 minutes. Rotate 1/4 turn to get cross hatched grill marks on surface and leave for 2 more minutes. Flip tri tip over to bottom and sear for 5 minutes with lid down. Shut off burner on 1 side and move meat to the side that is turned off. Close lid of grill and let cook for approximately 30 minutes for a 2 1/2 to 3-pound tri tip. Test meat for doneness and apply the barbeque sauce for last 5 minutes of grilling. Slice thin and pile on buns with additional barbeque sauce.