It is based on the Hollywood Park Casino in Inglewood, California. But also shows resemblance to the Garden Paddock of Santa Anita Park in Arcadia, California. It (along with the attached above-ground parking garage) is inaccessible to the player and plays no role in the storyline.
Elsewhere, players have created a current list of 54 countries that can't access Diamond Casino's gambling features. It's wide-ranging, including China, North Korea, Iceland, Cyprus and Poland.