This paper studies a control of the extending space structures in orbit under gravity-gradient environment. To this end, it first introduces an equation of motion of the satellite that extends on the orbit and shows that it is a linear time-varying system. Then it is represented as a linear parameter varying system and gain-scheduling control method is applied to control it. In order to design the non-conservative controller, the analytical method of constructing the smallest number of scheduling parameters is proposed based on a typical cylinder model. Finally, even when the generalized plant has the model error, it is shown to be applicable by adding the robust property with the constant matrix scaling. The availability of the proposed methods is proved through numerical simulations.