I would think so. I would go directly to a manager to be on the safe side though.

I have always been able to return on-line orders to physical store orders ...can't remember whether I have done that at Gap, but I feel very confident that you can.

I would be surprised if they didn't allow the exchange. I'd also return the other one you ordered that doesn't fit at the store (rather than ship it back). I just returned some items by mail. Even though there is free shipping both ways, the email I received to confirm the return said to allow two billing cycles on my credit card for the credit to be posted. Errr!

OP here: It could not have been easier. The store had the smaller size and returned the unwanted larger size dresses. Here's a picture if you are interested.

Here is a link that might be useful: Gap

It's adorable !! Great buy !