To go along with the free Destiny 2 and Samurai Shodown that you can already access, Google just announced that for the month of December you’ll be able to pick up Farming Simulator and Tomb Raider for no charge.
If you were one of the few that happened to purchase either game in the past week, Google has said you’ll be able to get a refund on those purchases. They made it clear that this is an exception, though, so future free games likely won’t result in refunds for anyone that previously paid for them.
Google hasn’t been definitive on how often free games will be released, but it seems that we can expect 1-2 free games each month. We’ll see what January holds. Also, remember that these free games are only for Stadia Pro subscribers. That’s the only tier that exists right now, but sometime in 2020 Google will release a free tier and those users won’t be eligible for these free games.