Just to clarify, I need to have all my GOG game installers/extras on one hard drive in order for gogrepo to work correctly, ie. see all my existing games to know which need to be downloaded. Hmm, I wonder if there is a way to join several partition on several (external) hard drives to one virtual one that the system would see them as one? Also in Linux (when they are NTFS partitions)? Then I could simply buy e.g. another 2TB hard drive to kinda extend the free room for my GOG games.
There are ways to do that in Linux, though there is, of course, the issue if one fails. (Also, other OS's won't be able to read such a hard drive.)
At this point, the ideal solution might be to make a dedicated file server to run gogrepo.py from. A Raspberry Pi will work, though not ideal; other SBCs are better for this purpose. (On the other hand, the Raspberry Pi has a bigger community, so it would be easier to get help with one.) If you have a much bigger budget, of course, something like a FreeBSD server using the ZFS file system would work. An in-between option would, of course, be to build a cheap computer with USB 3.0 and/or a big hard drive and use that.
By running gogrepo.py on a server, you can access it from computers running any modern OS, even if that OS wouldn't be able to read the underlying file system.