Is there such a part as an gas filler neck tube pipe attached to gas cap?

Yes there is such a part. It is the pipe that makes the connection between the filler neck and the fuel tank. As far as you just needing a gas cap it would have depended on the condition of this pipe. If the pipe had developed a hole or had become porous due to rust or whatnot it needed to be replaced. You might of just needed a gas cap but without seeing the old part you'll have to go on faith that you didn't.