Tesla is working on Steam integration for Tesla Arcade
However, Elon Musk announced recently that specific games may no longer need to be ported to the Tesla anymore. In a short Tweet, responding to a video showcasing the Tesla's gaming capabilities, Musk said "We’re making progress with Steam integration. Demo probably next month."
As pointed out by PC Gamer, Tesla has a shareholders meeting in that timeframe, so it's possible the feature will be demoed there rather than a public trial.
Not much is known about the exact specs of the Tesla's internal computer, but Tesla itself claims on the page for the Model S that it features "up to 10 teraflops of processing power" for "in-car gaming on-par with today’s newest consoles."