No gap. The gap is too small and you can’t clean between the wall and the vanity. Also if you would install a tower bar there, all the water from your hands will drip down.

If you're having the cabinet made, then no gap.

That gap is too difficult to clean and will be a dust/dirt/dribbled liquids collector. HOWEVER, if you are planning full-overlay drawers/doors on your cabinet, you will need a spacer in the gap so you can open your drawer/door without interference from the perpendicular wall surface. That spacer will affect the minimalist look of the front of the cabinet. If you choose open shelving or no storage, then you won’t need the spacer. I have yet to find a wall-hung cabinet that is made in North America and is not full-overlay.