First, make sure that you didn't break or damage any of the clips that attach to the top and sides of the rear bumper. Then double check to make sure that you have attached all screws and fully clip the bumper into all clips (especially the ones on both side, as there are two on the right side and two on the left side, I think). If the bumper is not fully clipped, it will cause a gap.

I might help to get a little more info. When you say "gap", where do you mean? Underneath the lights? Near one side or both sides? It is more of a sag (i.e. bumper hanging lower than normal)? A more detailed description might help us help you 8) .