Based on this thread question, basically the same post, with a request for help:

"Do you back up offline intallers for your games? Why or why not?"
https://www.gog.com/forum/general/do_you_back_up_offline_intallers_for_your_games_why_or_why_not/

Somewhere else I've answered this question with the "Yes." joke.

A situation happened though, worthy of explanation and maybe a request of help to other experienced users.
To explain, these are the steps I take before backing up my games:

1 - Download the offline installers using lgogdownloader
2 - Install the game and its dlcs if any on a fully offline Win 10
3 - Check installation folder for language packs or language files that I do not intend to use, requiring unnecessary disk space and delete them
4 - Use 7zip commands to hard compress the installation files and save even more space*
5 - Fully uninstall the game using a hard uninstaller tool like GeekUninstaller/BulkUninstaller to delete game files and, possibly, registry keys
6 - Extract 7ziped files and try to run the game to check if everything's working fine
7 - If yes, back up the compressed game files

(*save space = Results vary from -70% to -30% space saved with 7z, for example, Alien Isolation GOG setup is around 20 Gbs, counting DLCs, while installed game files go up to 35 Gbs. Hard compressing installed files with 7z, I managed to reduce it to 14 Gbs. I know there are even harder methods to save even more space with compression tools.)

...but this last weekend I went to backup two games:

Dragon Age: Origins Ultimate Edition
Alien: Isolation Collection

...and step 6 gave me trouble.

Let me explain.
Both game editions have DLCs, which GOG installers, by manual installation or by registry key registration tie to the chosen game installation folder.

In step 6, forcibly removing such registry keys or "making the game fully portable" if you will, broke the DLCs installed.

Using this method, Dragon Age Origins Ultimate Edition essentially became Dragon Age Origins, without its DLCs being recognized by the game, although their files are present in the game extraction folder.
Same way, using this method, Alien Isolation Collection essentially became Alien Isolation, without its DLCs being recognized by the game, although their files are present in the game extraction folder.

Rarely I play games with DLCs, so this was new to me.
I'm sure there's a way to use InnoSetup to recreate GOG installers with deeper compression managing to still make the game "portable" without whatever checks GOG setups do to make the game recognize the DLC files.

To be simple, I'm basically trying to compress game files even more and save even more space without breaking games with DLCs, but, apparently, GOG offline installers not only have trouble with DLCs requiring registry checks but they also require to be checked by the first game installation registry checks, making the game not recognize DLC files, even if they're inside the game installation folder.

Is it possible to turn games truly portable to save 30%-70% space using 7z?
Any ideas?

This is a solution for Linux only, but ./play.it already does what you are looking for.

Dragon Age: Origins is an example of supported game, that goes from 22GiB for the GOG-provided installers to 11GiB once repacked by ./play.it. Of course the add-ons of the Ultimate Edition are included and working as expected.

If you want to fix your own package, the files that are shipped in "support/userdocs" should be installed into the current user’s "Documents/BioWare/Dragon Age".

Thanks for reminding me of play.it. I might check the docs and the scripts later to learn to do it myself with other games too. It's not really only that, BUT your guide in the link you provided really helps to do what I originally wanted, so thank you very much!

The part that I really needed actually was to learn what registry keys GOG installers edit to make my own hard compressed installers using InnoSetup.

I also use InnoExtract most of the time, but, as you know, InnoExtract do not extract 'installer_script.iss', which was exactly what I needed to understand how GOG installers communicate with GOG DLC installers to mark such DLCs as applied in game.

Its a ton of work to do that with all installers... but saving space means saving money in the end.

By experimenting a little yesterday I noticed that GOG installers mostly use 'lzma2' compression in InnoSetup script / setup creator, while there's a stronger option: 'lzma2/ultra' which reduces setup sizes -50% in average.

I'd like to know why GOG team use lzma2 only. Maybe it is to save Ram and Time when users are installing?