If you’re not an admin, you’ll get prompted for the admin password when running the installer.

For example, with Postal: Brain Damaged, the installer gives you the option to launch the game at the end. The problem is that this launches the game under the admin account, not the user who actually started the installer. Then when you run the game again later, you’re left wondering what happened to your saves.

Is this a general issue with GOG installers, or does it depend on how the game is packaged?

I'm guessing you're insisting on using Galaxy?

I'd sooner use InnoExtract.