Have a wall that I initially thought was gypsum since it sounds hollow. However, I don't hear any studs when I knock on the wall, which I do in other gypsum walls in the apartment. When I drill (with a regular drill, not a hammer drill), there isn't much resistance. The material is dark gray, and there are no "voids." The apartment was built in the mid-50s. The plan was to put up an Elfa bookshelf. Could it be some form of lightweight concrete?
That variant of the ELFA shelf indeed sits less securely than the variant where you attach to the vertical tracks, as there are more mounting points. But a 6mm screw in lightweight concrete holds very well. There is also no direct outward pulling force. Therefore, with 6 screws it should hold for many 100 kg. I wouldn't hesitate about it.
Use regular plastic, if you use one made of metal that expands more, there's a risk it will crack, very porous material. As a bonus info: Don't try to screw near a door frame and for heaven's sake don't use frame screws to attach a new door frame 
(speaking from experience, the door casings and wallpaper held that wall together afterward).
(speaking from experience, the door casings and wallpaper held that wall together afterward).