I have from GG.
I don't know, that is what I was assuming on how it works. I remember it being a problem last year (or before ... ?). Since then I never really tryed it recently, because I'm hardly offline anymore. But from what I gather, they encrypt their downloads somehow and you need to be online for the "decryption", afterwards you can store the files indefinetly by taking them out of the temp folder.
But it could be very possible, that I am wrong.
Once the files are downloaded, when the first install screen appears, BEFORE doing anything else, copy the contents of the GG temp folder to a flash drive or other storage medium.
Once the game is installed, if you choose to keep the files, they get encrypted, but the encryption happens AFTER install.