Didn't get you....
A pic of that 'Gap' would be helpful.
Why? Because basically, the PSU is a standard one , not the slim kind nor the Tall kind, the case is also a standard kind, so they both ought to fit like a hand in glove.
The gap, as you have put it hmmmm, maybe, that's the gap that is supposed to be there between the side panel and the PSU because the side panel needs to hold on to something at the base of the case , that's the place where you'll either have notches for the side panel to lock into and slide or an edge over which one flaps locks down into.