Gamma-rays, x-rays, ultraviolet, infrared, radio-waves, microwaves and visible light are the same kind of wave i.e. they are all electromagnetic waves (radiation). They just have different frequencies. <br /><br />They are an expression of one and the same phenomenon: electromagnetic wave propagation. The properties of the propagation of electromagnetic waves (like for instance the speed of propagation) does not depend on the frequency. Thus, they propagate in vacuum with the same speed: the speed of light.