My parents are not observant. They bought gift cards to a store on Saturday morning. They then used these gift cards to buy something for me. I did not know it at the time and used the item. Now that I know, can I no longer use the item? If they want to use the cards again to buy me something and I know it, is that okay or not?

After the cards have already been bought this time, and it's not a common thing that they do for you, then you can use and benefit from the gift cards in both cases. Just ask them with sensitivity, thanks and appreciation, not to buy for you things on Shabbat.