This is always an important factor to take into consideration whenever you are applying for a co-branded credit card. The Gap credit card is a store credit card, and just like many great store cards before it, this card is not affiliated with any major payment network like Visa, MasterCard, or AmEx. What this means is that this card was made to be used only at Gap.
You may consider credit cards with cash back rewards instead. For example, the Discover it® Cash Back credit card is a good option. You will earn 5% cash back at rotatating categories on up to $1,500 in purchases when you activate (you should register each quarter to earn rewards). Plus, 1% cash back on all other purchases. Also all the cash back you’ve earned will be matched at the end of your first year, if you are a new cardmember. You could turn $150 into $300. This card has no annual fee and 0% intro APR on purchases (when the introductory interest rate is over, the ongoing APR will apply).