I also laid out the rubber matting this morning. This was originally used under AstroTurf pitches and came in four rolls. The marks on them is just sand that I will sweep off and clean up. I have laid them out to give them a chance to flatten out before I cut them and lay them tomorrow.
I also laid the rubber matting, im going to give the floor a scrub and see if I can get the marks off but im not to concerned, its a gym floor after all!!