Store credit / gift cards are best treated as their own bank or credit card account (or cash account). This case of a return is special, in that you need to enter a transaction for the opening balance that is basically a deposit (if you use a bank account) or a credit (if you prefer the interface for a credit card) - and which reverses the expense of the originally charged item. E.g., if the returned item was for expense account "Tools", then the deposit would refer to "Tools" and the memo list the returned item (perhaps even attach a copy of the receipt as an attachment).
Then, just enter purchases against the balance as electronic checks (e.g.) if you used a bank account, or as a charge if you called the gift card/store credit a 'credit card' account.
Because gift cards / store credits can only be used at specific stores, it is best to create a different account for each store involved in order to track them. My view anyway.
