HereForTheBeer: So I'm thinking of (again, not being a software tech whiz) a slice of the Galaxy code that handles the downloading duties without all the other stuff.
I'm still trying to understand what are those downloading duties that you'd be happy with. These are the different ways you can currently download your GOG games:
1. Downloading of standalone offline installers (and separate update installers, for those games that have them; e.g. Titan Quest doesn't seem to provide such while many other games do).
It is essential to understand here that it is not really important which client or tool you use to download these installers, you'll still have exactly the same installers (and update installers, if available). No matter whether you use Galaxy's backup function, or the old GOG Downloader client, or your favorite internet browser, or gogrepo.py, or or or. The tool might be different, but you end up with exactly the same installer files.
2. Letting a client (Galaxy) download and install a GOG game for you, Steam-style. Apparently the meaning also is that Galaxy can delta-update those GOG games that you've installed with it at any given time (not sure if Galaxy is currently supporting this). In this case you don't have the standalone offline installer, the Galaxy client dowloaded and installed the game files in a different way. Of course you can still download the installer too, but you don't need it for the game to function, it is already there.
So when you say you want a lightweight client that does the download duties, are you referring to 1, or 2, or both?
Regardless of that, if you'd like to download e.g. the Titan Quest offline installer with that client, you'd still have to redownload it fully each time it is updated, as long as GOG/the publisher is not offering separate patches for the offline installer. However, the client might be able to delta-update your existing Titan Quest installation (
not the installer).
The point of this thread, and the related wishlist item, was those offline installers which sometimes don't get separate update installers. So even if there would be a lightweight client that can achieve 1 and/or 2, the original problem would still remain: no small update installers for e.g. Titan Quest.