Continue the friendship if you like, but discontinue the gift cards.
If they ask, you can be PA and say you realized you were imposing on them by giving them things they didn't want, and since they were uncomfortable with asking you to stop, you spared them the discomfort.
Or you can just reply that you have decided not to give them gifts the next time they ask where their gifts are.
If the friendship continues when you aren't giving them gifts, then they aren't using you. If it stops, then you know where you stood.