Do they have a gift shop?

Yes they have it

I was there at this time last year and they had a little case inside with some trinkets like shirts and coffee mugs.

I've been twice and don't recall seeing a "gift shop". Like Brian said, there may be items for sale but no official "shop". I was just there on may 27th (2019) and didn't see or hear of a gift shop.

It's more like a gift tent outside of the castle. They do have some other things like t-shirts and swords and a few other things inside though.