Not every gyratory is a roundabout, but every roundabout is a gyratory. That's about as far as agreement on the terminology goes; many people use 'one-way system' as the most general concept, and 'gyratory' is often felt not to be understood and to be too abstract. I tend to use 'gyratory' or 'one-way system' only for non-roundabouts and 'roundabout' for roundabouts, which is really a fairly roundabout way of saying that I call a spade a spade.
Not every gyratory is a roundabout, but every roundabout is a gyratory. That's about as far as agreement on the terminology goes; many people use 'one-way system' as the most general concept, and 'gyratory' is often felt not to be understood and to be too abstract. I tend to use 'gyratory' or 'one-way system' only for non-roundabouts and 'roundabout' for roundabouts, which is really a fairly roundabout way of saying that I call a spade a spade.