Hi Judith,
Welcome to GCU!
GCU tried a 'gallery sub-category' setup in our stores, but it works very poorly, is no longer supported and not recommended. So no, there is no way for you to categorize your cards within your store.
As a veteran of the site, I can say that it really doesn't matter. If your keywords and meta-tag data are solid and you've followed all the reviewer's advice and instructions during the review of your cards, they will indeed be found- both onsite and off.
You can play with the sort order of your storefront and you can create up to 25 cards to be in a featured section of your storefront.
This is a helpful article on the Community Blog explaining how to use these tools:
https://gcucommunity.com/2017/10/06/nuts-and-bolts-storefront-organization/ Good luck with your new store!