I have the founders addition and stadia pro, which lets you pick a free game every month, I got to choose a game in November but I haven`t had that option in December.
So when do I get to choose this months free game?
If you're a founder you should have been able to claim 4 games... Destiny 2, Samurai Showdown, Farming Simulator, and Tomb Raider Definitive edition. Check the store and see if they are still claimable. I can't tell from my store because I've claimed them all. It's possible you missed out, but I'm not sure. You can also chat with a Google rep to find out if the ability to claim them has passed. But those 4 games were all made available to Founders.
On January 1st, another Tomb Raider game and Thumper will be available to claim.
Also the Tomb Raider game for January is going to be Rise of the Tomb Raider: 20th Anniversary edition. Which I'm glad because I bought the Shadow of Tomb Raider definitive edition when it was on a pro deal. So I'm glad that's not the one they're giving away now. I guess I'll have all the Stadia tomb Raider games now, lol.
It was announced in the community section here :
My wife got me the "Premier Edition" for Christmas and I also had the option to choose the same 4 games you mention from the Founders edition.
@NadeFodder So far there was 2 free games in Decembre and 2 more coming in January Thumper and a Tomb Raider. You don't need to chose one just to claim them in your Stadia app or on the Stadia website in your chrome browser.