I just got the game and wanted to download it, but when the 2 GB file reaches about 50%, it stops ans says : unable to read source file
This happened several times on different computers.

Sounds like the problem lies at GOG.com, if the problem occurs on different PCs.

I suggest you go to the support and send them a ticket regarding the problem. You can find it here:
http://www.gog.com/support/contact

Or are you using the GOG-Installer? See if there is an update. I had download problems, too, when my installer wasn't up to date.

Edit: Und jetzt sehe ich, dass du auch aus Deutschland kommst. Mensch, da hätte ich mir den englischen Text auch sparen können. ;)
Wie gesagt, wende dich an den Support. Scheint an der Seite zu liegen, nicht an den rechnern. Da können wir Spieler wohl weniger tun.

I use the normal download from mozilla and exept now I never had problems downloading from gog.
Sometimes there was the problem that when I downloaded a file on one pc and it works and I copy the file to another pc and it says the file is corrupted. But it worked when I downloaded it with the other pc.

PS
Ja ich bin auch aus Deutschland.
Lustige Dinge sind am besten in der Originalsprache.

Finally it is finished.
I needed to run both mozilla and gog downloader in administrator mode.
Even this took some tries but it is finally done.

Maybe there are some problems now because everyone is downloading due to the "end of the world sale".

Ok my problem is download just stop at 3.14 GB, it saying download but saying
nothing