I wouldn't replace things my kids lose after they insisted on taking it and keeping it safe. Eg my 4.5yr old takes toys to childcare all the time and occasionally loses it. He knows that if he loses something it isn't getting replaced. I don't want my kids to think that they can be careless with their things and not have consequences. Even though I could afford to replace the gift card, I wouldn't. Someone above said kids have enough life lessons but I think this is one of those ones that if you give in one time, your kid will not believe you mean it next time. Maybe this is tough parenting for some.
In this situation, I can see how my son would have insisted on wanting to hold it himself, and if he had lost it I would say he isn't getting another card. But I might tell him if he does "x" (some good behaviour) I will get him something else later, like play dough or a book.