What game? Do you have a screenshot of the directory with the files you’re trying to install?

Did you download all the installer files?

Guessing you're missing a file or one of the files didn't download correctly.

Some titles have so many files for the offlline installer, sometimes the first time you display the offline files (depending your browser settings / extensions or how GOG handles the "classic" version of the library) not all files are displayed

Depends the resolution of your screen i guess too. (especially with a laptop)

See The Evil Within + DLC for example, at first only 7 files were displayed for a resolution screen : 1600x900 (on a laptop) : that's a problem at first.

Above explanations (from others) are also possible.

Usually the product page mention IF the game requires Galaxy (it's usually most of the times for Muliplayer, or if you care of the trophies).

Nope. Ensure you have download all the files, completely, to one folder. Then you should see one exe and several bin files in one directory. Run the exe and it should work. If not you might have a corrupted downloaded file. If it continues after trying the file a couple of times, check with support. Also tell us the game, we may be able to try to download.

As other have already asked, for which games do you have this issue ?

Sorry for late reply. Neverwinter Nights 1&2 ,ATOM RPG Heroes of Might and Magic V.

My friend had the same problem on his computer as well , with his games . When he went to the high seas to get a GOG copy and download of his games, that he owns, as a test, they downloaded and installed with no issues .

Again, through GOG galaxy, the games install no problem. Its only when using the offical offline downloader that issues occur.

This last error was on Jade Empire

GOG downloader is no more. Perhaps that's the problem. I'm pretty sure everybody assumed you were using your browser to download those installers.

I was where it says"Download offline backup game installers". I was trying to use that, through the broswer

You say that Neverwinter Nights 2 is one of the games that has the issue. NWN 2 has 3 files. The first has 34 MBs, the second has 3.9 GBs and the third has 3.38 GBs. Are you sure that your NWN 2 files are the same size and are also in the same folder when you attempt to install it?

So, let's say you were downloading Neverwnter Nights. In your web browser you went to you account, selected NWN from your library, started downloading installer files (marked in the attached picture), and waited until the download process was completed. After that you tried to run the game installer but it showed you an error message?

I just downloaded NWN 1, the two files, and it installed fine on one of my Windows partition on which Galaxy was never installed. Are you sure you downloaded all the files ?

And when do you have this error message ? in the installer options did you try enabling "Check file integrity before installation" to make sure your files are not corrupted ?