PhysX is a software engine that game developers use to bring realistic physics to their games. In a normal retail install of the game, this software engine is installed on your system. However, in the GameTap Unlimited Play environment, these games don't go through the normal install process. As a result, any game requiring the PhysX engine on your system may crash or not launch when played through GameTap and you have not previously installed the PhysX software package. Get the software here.

List of GameTap games requiring PhysX:
Frontlines: Fuel of War (coming soon)
Race Driver: Grid
Gothic 3
Infernal
Shadowgrounds: Survivor
Sherlock Holmes: The Awakened