.Keys: If you can bash script, to write a little script that extracts and then compress your Linux GOG ports using 7z, PeaZip, or anything really, is easy and can save you a lot of space. :)

.Keys: I do not know for certain how they do their .sh setup installers, only that they use MojoSetup and that the compression is not that good. But I've come across a Python script (Therefore, easily usable on Windows too) a while ago on GitHub that allows us to extract them and see its contents without "installing/extracting" the game as intended:

_Auster_: If you have Windows Subsystem for Linux set up in your Windows machine, while in the folder with the installer through the terminal, you can use the command unzip gog-setup-file-name-goes-here.sh to extract a given installer, and either unzip -Z1 gog-setup-file-name-goes-here.sh >> files-list.txt or zipinfo -1 gog-setup-file-name-goes-here.sh >> files-list.txt to list files in the installer, with the latter 2 commands creating a "files-list.txt" file which will have the files from inside the installer listed. Using the command through a Linux virtual machine should also be possible.

.Keys: I do not know for certain how they do their .sh setup installers, only that they use MojoSetup and that the compression is not that good. But I've come across a Python script (Therefore, easily usable on Windows too) a while ago on GitHub that allows us to extract them and see its contents without "installing/extracting" the game as intended:

https://github.com/Yepoleb/gogextract

And an explanation on how GOG Linux setups work, by the script creator:

https://yepoleb.github.io/blog/2016/10/09/how-the-gog-linux-installers-work/

It worked with everything I tested it in. (Practically all the Linux ports on GOG of games I own.)

If you can bash script, to write a little script that extracts and then compress your Linux GOG ports using 7z, PeaZip, or anything really, is easy and can save you a lot of space. :)

Bladeforce: Lets put it this way, I have made myself and used for a few months now appimages from my GOG collection without problems.
I now have over 200 appimages of all games i own. No problems whatsoever

Timboli: Thanks for the info.
I'm not sure I was aware of it, though I may have been and dismissed it because there is no List or Test option.
7-Zip works well enough for me, with the correct command-line usage.
In any case, I am more concerned about the MD5 value and size check.

.Keys: What command line you use for 7z?
This is what I use:

7z a -t7z -m0=lzma2 -mx=9 -mfb=256 -md=512m -mmt=2 -mmc=10000 [archive name.7z] [file or folder to compress]

But I do not have so much knowledge in compression.
Still, this worked well with good results as compressible files have -50% to -70%, and in some rare cases, -90% file size compression with those commands.

Timboli: It seems we have a misunderstanding, I was talking about checking SH files, not compressing.

These are the two commands I use.

(TESTING) 7z.exe t -tzip "$path"

(LISTING) 7z.exe l -tzip "$path"