On August 20, GameStop sent out a memo saying that, effective August 21, 2019, they would hard-code against GC-to-GC conversions.
They had previously hard-coded against buying Amazon gift cards, and now you can no longer buy any third-party gift cards at all. People used to buy discounted GameStop gift cards first, then go to a store and convert them into Amazon gift cards, and later into Home Depot gift cards, making a nice profit along the way.