Luckily not every game on GamePass is a Windows Store (Universal Windows Platform or UWP) game though. Whether a GamePass game is hookable largely depends on whether it is a classic Win32 program or was programmed as UWP app. Most of Microsoft’s own games for example are UWP apps and thus protected against hooking into them while most third party games aren’t.
On a sidenote: things *might* improve for future UWP games. I don’t follow the whole topic overly closely, but my impression is that MS recently is changing course a bit as far as UWP is concerend. Many developers (and game developers in particular) never were comfortable with the restrictions it brings. So at least there’s some hope that future UWP games might not be sandboxed as restrictively as they are now.