There's no way to get Gamestop or Best Buy credit from WM directly. The only way is to flip from WM to them but that never happens.
When I get stuck with WM credit, I just buy gift cards from their gift card mall: Taco Bell, Starbucks, Red Lobster/Olive Garden, and Outback Steakhouse. Some cashiers may deny you so just go scan the gift cards yourself at the self checkout registers. I hate shopping at WM.
iirc, store credit only works on the prepaid CC's that have fixed dollar amounts. The variable prepaids are coded to not accept gift cards.