You can watch the CGI Movie adaptation that gives you the general idea of the Gantz IP.
If you have Netflix it's:
https://www.netflix.com/search?q=gantz&jbv=80149259
I purchased Reika as soon as they announced her, which at the time only the Black were available/announced. I've been wanting to complete the set with Anzu for a while now, and I've been wanting Persona 5 Joker since they launched him but they've never had a viable sale for him. So getting the two'ffer deal was a no brainer and pleasant surprise.
I've unboxed Reika when I purchased her and I can tell you she is amazing, if you like the IP this offer is great for both.