I am currently downloading "The Witcher" for the fourth time!
First time, using the latest GOG downloader, the result failed the integrity check. Please redownload!
Second time, using the Web Download in Chrome, the result failed the integrity check. Please redownload!
Third time, using the Web Download in IE9, the integrity check passed, but the installer complained about a missing file, Please redownload!
Now I am trying the GOG Downloader again, but honestly: Am I doing anything wrong? Any ideas?
I don't want to buy this game (or, for that matter, "The Witcher 2") from Steam, but the situation is getting ridiculous...

Check if your AV / firewall program does not interfere (or just disable it for that one download).
When using browser, remember to get all files in one place before running the setup (and run it with administrator rights).

Unfortunately, from the description I'd say the problem is almost certainly on your side.
Alternatively, post the size of downloaded files (.bin / .exe ), maybe you'll need to re-download only one of them.

let's see... the files were all in my \download folder, I did run the installer with administrator rights, so that cannot be it.
How a firewall or antivirus program could corrupt a download is not quite obvious to me (and I sure cannot be the only one who uses the MSE), but I'll try...
While we're at it - the currently running GOG downloader shows a little vertical gap in the download bar (see image), and I remember the last time I had a corrupted download, there were several of these as well...
Is that an indication of a corrupted chunk? And if so, what can I do to redownload that specific part?

Fascinating indeed!
Looking into the "witcher" download folder for a few minutes, a number of chunks were simply stuck! Some had not even started, some were frozen in mid-download.
After finally managing to deactivate all firewalls in my network, both local and on the router, they started up again...

I'll keep my fingers crossed, while the download finishes. Thanks in advance, Thiev!

More info: Even now, sometimes chunks get stuck! I have to cancel the download then, and re-add it from the GOG website before it continues properly.

Weird indeed...

Good that re-adding works, although it shouldn't be necessary in the first place.
Quite possible it is caused by server strain, due to all that TW hype at the moment. Or something on your side prevents the downloader from working properly. Anyway, if in the nearest future it will still continue with different games, you may consider contacting support.

And the story goes on: The downloader "finished" its job - at least that is what it claimed, since there were incomplete chunks from all 5 .bins left!
Nevertheless, it managed to combine these somehow and started the installer - which of course halted immediately, since only one bin had been built properly.
I went back as always, deleted the download from the downloader and wanted to add it again so it could fetch the rest - but, alas, now I cannot add a download from the website anymore, the Downloader just sits there, its queue remaining empty!

As it is, I have by now downloaded 35 Gigs over the last two days, have not been able to play the game I paid for, and wasted several hours fiddling around with all this!
Disappointing, to say the least. I really do like GOG, and have bought from them before, but it does not look like their infrastructure can handle releases this big - "The Witcher 2" will have to wait, I am afraid (>_<)

At last!
The Downloader eventually sprang to life again, and after having only to download bin2 once more, "The Witcher" finally installed...
Thanks for your patience, Thiev, now I am off to play (^_^)

Unsolicited approval: this works. I had "setup_the_witcher_enhanced_edition" and "setup_the_witcher_enhanced_edition[1345]" and two hundred CHUNK files for part 2, so I hit the X on GoG Downloader to delete it from the queue, quit, restarted it, and added The Witcher again.

There was a tiny sliver of not-green on The Witcher's download, and after a brief period it showed completion (again) and this time passed the file integrity check.

N.B. I simply succeeded in downloading the thing by putting in 101% effort. I haven't actually installed and run it yet, and looking at the support forum, it doesn't matter if I did because it could eat itself at any moment. But that's computers for you. Non-re-downloading via this method, on the other hand, works.

Mac OS 10.10.1
Got 1.7 Gb with Chrome download, then crashed [middle of the night to encounter less traffic]
2nd try got 3.67Gb with gog downloader, then crashed [early morning to midday]

Speed never got above 267K/sec. I have 1.6Mb/sec connection.

No error message, just the spinning beach ball of death.

gog tech contact would not allow attachment of Mac .spx format system report, so it would not send. With no other contact options, here I am, 26 bucks poorer and no game

EDIT: Zero-th of all, welcome to GOG!

First of all, posting in someone else's question thread that was marked "solved" well over three and a half years ago probably isn't the best way to attract possible help. ;)

Second, I can't be sure from your post: did you actually try contacting GOG Support itself and filing a support ticket, or were you talking about some automated "send error report" function built into the GOG downloader? (I've never tried the downloader, so I know little of its features and functions.) If it was the latter, then click my link and do the former. :)

I don't really have any ideas to directly solve your problem, unfortunately, other than suggesting you keep trying to download on different days. I've occasionally had issues with game download speed here, myself. It seems to vary based (presumably) on things like server traffic. Besides filing a support ticket (and continuing to follow up on it), if you still have no luck, you might consider starting a fresh thread in this forum. Be patient, but don't give up.
Sorry if this wasn't much help.

P.S.: TWENTY-SIX dollars?!? How many copies did you buy?!