rouliann: Would you be kind enough to tell me where a similar tool exist then ? :-)
timppu: Many (including me) use gogrepo (it is actually currently downloading updated and new installers and extras for me, still 374 GB to go :)):
https://www.gog.com/forum/general/gogrepopy_python_script_for_regularly_backing_up_your_purchased_gog_collection_for_full_offline_e/page1 Then there is at least lgogdownloader, mainly for Linux users I think (albeit I guess one could compile it themselves to e.g. Windows from the source code?):
https://www.gog.com/forum/general/lgogdownloader_gogdownloader_for_linux/page1 There was also a third one someone released maybe one or two years ago, but I've forgotten its name.
Anyways, the more the merrier. It is good there are several different options which might serve a bit different needs (e.g. lgogdownloader appears to have more features than gogrepo, but then gogrepo is more portable, running on many different devices and OSes as long as they support Python 3.x), and of course it tends to be some tools might not be that actively supported by their author anymore, unless some others start contributing to it in e.g. GitHub.
E.g. the gogrepo author still appears to be developing his script and people are eagerly waiting for a new release which is supposed to e.g. speed up the file details download from GOG servers a lot, but it seems he has been quite busy with his personal life lately. Hopefully he still finds enough time to release the new version.
Thanks a lot for all these useful informations.
Now I feel a bit dumb for not having found these tools before... they look amazing! (and a lot more complete than my little script) :-)