Purchased an item at best buy 6 days ago, now they're offering a $50 gift card

I would think that they would just give you the gift card.

You're going to have to go there either way, so just go ask.

Bring your receipt to the customer service desk and tell them that you want to do a "return/exchange". They'll scan the receipt one time for the return, then another for the exchange, and they'll then be able to give you the gift card.