I don't see where there would be a problem to do that. It is actually easier for people to just get a gift card than going through some of the registries and trying to find some of the items on the list. I think that sounds like a good idea. Good luck with your baby shower!
I think it's perfectly fine. The parents can use the gift cards to buy exactly what they want for the baby, instead of having to rely on the guests to buy stuff they think the parents will want, like clothes, toys, etc.
I think it is ok to give a gift card escepelly if you don't know what to get as a gift .