I'm getting my braces off tomorrow, but I have a small gap between my two front teeth. Will my retainer fix this?

Gap. Possibly. Before your braces are removed please discuss this with your treating Dentist or specialist Orthodontist.
Incomplete result. if you're not satisfied with your end result of orthodontic treatment because you still have a gap between your front teeth you must communicate this to your treating dentist. He should continue treatment until your satisfied with the end result. .