Can you fix a gap in your teeth with a retainer?

Depends: Retainers "retain, " they hold teeth where they are, that said, there are removable appliances that can reposition moderately malposed teeth. The question is why do you have a gap? Are teeth shifting because of gum disease, malocclusion, or do you have a tooth-size discrepancy. Please see a qualified orthodontic specialist for evaluation.

No: A retainer is an appliance that holds teeth in their new position after braces. There are other appliances that may, but they will usually need braces in addition.

No but: There are other methods and appliances that can. However, you need to see your dentist and discuss what options would be most appropriate for your particular circumstances. While it seems like a simple question that could be answered simply, the solution often requires the consideration of many factors too complex to discuss on this type of forum.

Retainer / braces: You may qualify for invisalign which looks like a retainer but moves your teeth too. It also may be able to be done in a short time if its a minor movement. Ask you dentist for an evaluation.