Always nice to spend a couple of days downloading 25gb of files just to run the installer and be told that "a" file is corrupt. Which one, of the dozen or so? No idea, as the machine certainly isn't going to tell me. Then to redowload the installer file, run it, and be told that it cannot find the files, even though it's in the same directory. Yeah...I'm not doing this again.

You are talking about using http to download the offline files? If so, double-check that none of the filenames are wonky as if a file has been downloaded twice.

Dowload installer from GOG Galaxy > Game > Extras section. It will far more likely download correctly. You don't have to use GOG Galaxy for any other reason but to download the installer. I expect it t has its own automatic file verification for these as well as it probably is applied to all downloads.

When downloading from a Web Browser, it can do things like randomly cut a file short from any connection issue or anything else. Like Alcaray says, things can easily go wrong.

If you have low internet cap problems, GOG Galaxy would be better for you.