Sorry, we only have direct downloads from the website available for the latest SKUs of each title. But the GameStop app is just the latest version of Impulse (they bought it a couple of years ago) which you would have used originally when you got Entrenchment; you should already have an account there the same as it was then.
If you have any trouble with it, please email support@stardock.com and they should be able to provide some alternative.