If you use number of prints per page as your sizing option, it'll split the page to fit exactly that many so there will be no room for a gap.  For example, if you have a page that has 8x10 printable area and you tell it to split into 2x2 rows/cols, then you'll get four "cells", each at a size of 4x5.  In other words, when you split a page into rows/cols, there is no space left for a gap.

BUT... You can basically do what you want using a border.  Continuing with the above example, if you wanted a .5 inch gap between prints, you could add a .25 inch B (not B+) border and that would basically "shrink" each print so it has a .25 inch border on each edge.  I believe that'd give you what you are looking for.  There are many ways to do this.  Another would be to create a reusable custom layout that does the same thing.  But I think the above is probably the simplest.

Regards,
Mike