Mine isn't the Trex brand which is about half again as expensive. Same stuff though. I used a small finish nail for spacing, not much of a gap at all. not even what they recommend but it's enough for water to flow freely through which is all I wanted. Stuff isn't cheap but never will I be out there sanding, staining, sealing. Pressure wash it a couple of times a year and it's good to go. Wouldn't need to wash it that often except for it being uncovered and of course muddy paw prints.
No exposed screw heads on mine. It has a cut down both sides and a stainless U clip goes into the edge of the first board, then a stainless deck screw through the clip and into the joist. Next board slides up to the first and the other side of the clip goes inside that edge. Neither the clips or the screws are cheap but then rust is not your friend. The deck is remarkably solid and quite permanent, except for the pressure washing forever maintenance free.