Don't do this shit. You don't know why that person is returning donations for gift cards. Did their child get sick, and they can't afford medicine? Will that gift card pay for 2 month's worth of food? They're in their own right to do whatever they want.
Don't police poor people.
This may upset some more “traditional” people but I think you should live together before you get married. There is a whole different dynamic in your relationship when you actually live together vs when you have your own places and choose to sleep over a lot.