I think I sold a woman a faulty gift card
Why did you think the gift card was faulty?
I was told by the other person it works, but I don't think it did.
It's wrong to provide someone with a faulty purchase. You should not listen to others when you are ringing up a purchase. Instead, you should've double checked that the card is properly activated.
I handed the money to her but the gift card didn't work.
Double check in the future to see if the gift cards are working properly and don't feel rushed by the customers or your coworkers. Measure twice and cut once as they say, it is faster to double check than to make a mistake.