In case anyone has this issue and wants to know a workaround, you can resume a failed download from your account without using the GOG Galaxy app. I'm not sure if it works on every version of Windows, but it worked for me on Vista and 7.

Once a download fails, go to the directory where the failed file is located which will have an extension as PART on the end. (i.e. randomgame.exe.PART)

Copy and Paste or Move this file to another directory/folder.

Go and attempt to download the file again from your account and up in Firefox's right-hand corner there is a download icon you can click on to show you your download status. (There may be similar icons in other browsers, not sure.)

Pause this download, go to the saved failed file you had previously copied/moved and copy it again.

Paste it into the directory with the new download attempt and overwrite the new file with the old.

Now go back to the icon on Firefox, click resume and it will resume back at where the file had failed previously, saving you time on downloading, especially if it is a very large file.

or Download using download manager and if download fails simply refresh download address and resume download.

Not sure if it will work 4 me as nv happen 2 me yet but thks 4 the tip! :)

Yeah I was aware of that Firefox workaround for resuming downloads, good to know it apparently works on GOG too.

However, that workaround doesn't always work on some sites. At least on some sites if the download doesn't reach the end, the browser still seems to think the download is finished, even though you got only a part of the file. There is no .PART file anymore, only the original filename with a truncated file.

So it may be a case by case thing. Maybe this works for cases where the internet connection dies abruptly (you'll have the .PART file), but sometimes it apparently may be the server which decides to cut the transmission, leaving you with a corrupted file that can't be resumed.

Hmmm, then again... it didn't occur me to try what happens if you rename that truncated file to add the .PART in the end of the filename? Does it resume then? I'll have to try that if I ever face the problem again.

It's also possible to use a tool such as wget or curl to resume downloads. (Of course, if you want to use it on gog.com, you will need to provide the program with the cookie that says you're logged in. Or at least I think that's how it works.)

On my Linux system, I actually prefer using wget for large non-gog downloads (like .iso files of Linux distributions).

Oh. Interesting trick in downloading using Firefox. I just know about this. Thank you for the tips.

Or download the way I said when download manager is a glitchy piece of crap and doesn't work for some users.

DownThemAll!

Works great with GOG on any OS with Firefox.

It works every time for me and never any problem occoured with gog files or humble bundle files.

i'm new to GOG i purchased witcher 2 don't know why download failed and now i cant resume the download plz help me guys ( sorry about my English )

it does not work anymore, it now changes the name of the part every time you try to redownload one.

i.e: ijklbdpart2 when you download again will be called bjkdiouspart2

Therefore when you redownload, what you have to do on top of this, is:

picking up the name of the new download part "bjkdiouspart2"
copy it, and rename this part to some random name, and copy that name of the previous attempted "ijklbdpart2" which now become "bjkdiouspart2" and now you can resume your download...