Quoted from FGF subreddit:
You get Gears 5 Accessibility Preview added to your account, which is in fact the same as the full game and you can play the whole game including multiplayer. This has been active since July 29 (so for over 3 months already). There is no end date in sight, so enjoy the game!
Original post by u/Agile_End with all instructions that are needed:
This will likely NOT work for people who have Xbox Game Pass subscription.
If at any point something doesn’t refresh/show up, try closing/reopening the apps; make sure you’re signed into them with your Microsoft Account.
The Xbox Insider Program allows you to check the latest system updates and game features. The Xbox Accessibility Insider League allows users to provide feedback on accessibility to game content and features. Here’s how to use them to get Gears 5 for free.
Step 1 - Install the latest version of the Xbox app and Xbox Insider Hub:
Step 2 - Launch the Xbox app and sign in with your Microsoft account. If everything is updated, a shortcut to the Xbox Insider Program should appear on the left side of the bell icon inside the app.
Step 3 - Click on the Xbox Insider Program icon and sign up for the test program, then join the Xbox Accessibility Insider League (in the Previews section of the Xbox Insider app).
Step 4 - Find and join “XAIL Gears 5 Accessibility” (which should now be present in the Previews section of the Xbox Insider app), which will add Gears 5 to your Microsoft account for free.
After the game gets added to your account, you can leave both Xbox Accessibility Insider League and XAIL Gears 5 Accessibility programs; Gear 5 will stay in your account.
Gears 5 supports Xbox Play Anywhere (PC / Xbox One / Xbox Series X | S).
If you need a guide with some pictures, this website copied the above guide and included some pictures: FREE Gears 5 (Xbox Play Anywhere)