the metal cage needs to be put in, you bend the tabs on the cage so that it clips onto the radio surround. then you put the head unit in that will clip into the metal cage and stop it falling out.
the pocket above it looks to be the wrong size, that is why you have a gap.
you can buy the pocket from vw for about £13. also there is a plastic V shaped part that goes between the two, not entirely sure what the use of this is, maybe just to provide a small bit of support for the pocket, im not sure, mine got swiftly removed and my double din fitted.