Basically, I'm on the edge of my wifi network, and so the connection drops for a short while now and again.

"But what does that have to do with GOG Galaxy ?"

Well, the web installer, doesn't retry the connection like 1 or two seconds or so, it just goes "Bye !"

I think someone who made the 'web installer' (since it's halfbaked), should restart with the basics.
(program flow)
When programming a web installer (no quotes), always make sure it's able to finish the job,
instead of the client of your shop retrying time and time and time again, with zero result.
If the connection fails, especially for the GOG Galaxy download, make it retry, not exit.
Even a halfbaked programmer should be able to do that.

TY.

It sounds like you want something like LGOG or one of the other utility programs that trounces Galaxy in both function and sanity.

And/or a powerline adaptor/better router/repeater.

Another alternative suggestion to Darvond is to get a WAP. Use an old router and put it in access mode in the same floor or room you're in for a stronger wifi signal.

Here are links to the latest full installers. The first one is for Mac and the second for Windows. Presumably your browser is able to resume the download if it's interrupted. Once you have GOG Galaxy running, I don't know how great of an experience it's going to be with a spotty connection.