Typically, you will not receive cash back on your purchases. Your Gift Card is a prepaid debit card. Every time you use it, the purchase amount is deducted from the available balance until that balance reaches zero. For example, if your available balance is $50 and you buy a $40 shirt, the $10 difference will remain of you card and become your new available balance.
Sometimes, in the case of returned items, a merchant may provide a cash refund.