First of all, this Xbox app is still a joke. What is more terrible is that you can't get proper troubleshooting from Microsoft staff, and you have to wait for a player to come up with a useful solution.
Rant off**
If you can't install Gears Tactics via Xbox Beta app and you are logged in, and you have an active subscription, do the following:
1) Check for windows update by opening up Start menu and start typing 'Check for updates'
a) If you see 'You're up to date' you are good to go. If Gears Tactics still won't install, jump to 2)
b) Install updates, restart computer. Re-check for update, install update, restart computer, etc. until you see a)
2) If your system is up to date but Gears Tactics still wouldn't install, visit https://www.microsoft.com/en-us/p/gaming-services/9mwpm2cqnlhn and install OR update Gaming Services
The above steps helped me finally installing Gears Tactics. Good luck
