what is the significance of carlson asking geogre and lennie had his gun and supplying the story that george wrestled the gun away from lennie and shot him

George is the one who took Lennie's gun. Things would have been much more complicated had the actual truth come out. George would have been interfering with the justice Curley wanted. Also George's act of compassion (shooting Lennie) might not have been interpreted the same way by others.