I had previously problem with corrupt files on other games-and it was impossible for me to download my games.
What was the solution:
1. Used the latest version of GOG Downloader.
2. Disabled temporarily my AV program - I use Avast. Make sure that the AV is disabled for the whole time of download - on my first download of Fahrenheit I made a mistake and disabled it only for 10 minutes and I had the corrupt message only at the end.
3. If that happens then delete the game from the Downloader window. Exit completely the downloader, go to dowload directory and delete all of the already downloaded files - prefferably the complete folder of the game. Then restart the download with the AV disabled.
I hope it helps. This worked for me.