You can!! The only catch is, you need to ensure the total value of other items in your purchase is of equal or greater value than the gift card.
E.g. When I go to Superstore and purchase $100 of groceries, I also get a $100 gift card. When I go to pay I use 100,000 points (which to me is to cover the gift card) and then pay $100 cash for the groceries I intended to purchase anyway.
Hope that helps.
It was nice in the past when I could just walk in and use only points and buy a GC but a lot of people had points stolen so they stopped that process. It could still happen but this is more work so I’m guessing less likely to have it happen.