@osaman,
If you have your app config set to allow users to upgrade the app that is expected behavior. GlobalProtect doesn't disallow installation of the 6.0 or 6.1 clients on a Windows 8.1 client, so the upgrade will be allowed to install. On these older clients it may break things.
Assuming that you want your supported clients to still get upgrades directly from your portal like most customers have things configured, you'll need to create a separate agent configuration that targets these outdated machines and set the app config to disallow upgrades.
The most direct way of accomplishing this is simply targeting the user or device as it's the easiest way to set this up. However, through 'Custom Checks' on the config selection criteria as long as you have a GlobalProtect subscription you can make this more dynamic. You'd want to add a custom registry check(s) via HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion and find a corresponding value. If I recall correctly, prior to Windows 10 you should be able to read 'CurrentVersion' and it should read 6.3 for Windows 8.1 and 6.2 for Windows 8.
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