I think that contact support means actual support, rather than the forum. However, my suggestion would be corrupted installer and that a re-download might be in order.

This is probably a trouble with the GOG Downloader. Try to disable it in your account, then manually download all the installer files one by one (see the attachment). All the files need to be put in the same directory. To install the game, just run the first executable file and you're done.

There's already a thread about the same error on GOG.

You could also try to clear your browser's cache and sessions. There's a very good application called CCleaner that handles this task nicely, and it supports all common browsers (Firefox, IE, Chrome, Safari and Opera). It also clears all Windows temporary files, another possible cause of your problem.