LitRPG is a very new genre, the term only being coined in 2013. As such, it doesn’t have an official category on Amazon, and unlike grimdark, I don’t think it has enough mainstream support that it will get one. However, you will find plenty of self-published fantasy books with litRPG as part of their title or subtitle. It’s a sought after keyword, and you’ll find litRPG titles in high spots of other fantasy genre’s