There’s a lot of discussion about how to use them over on the Disney Gift Card Deals thread. Basically, they run as a credit card only at the specific brands listed. I’m not sure about Universal Studios, but a lot of folks buy the Happy Guy card to use at Home Depot as “credit” (don’t run it as a gift card, because it’s not coded as a gift card and Home Depot doesn’t accept GC>GC anyway) to buy Disney GCs, or you could pick up a gas card.
The Happy Lady card works the same way at Bed Bath & Beyond, which also sells third-party GCs, but BB&B probably has a smaller GC selection than HD.