It says that "the source file is corrupted" and that i should run the installer's integrity check to see which files need to be redownloaded. i used gog downloader...

Yeah that can happen, even with the GOG downloader. Check the integrity with the installer and re-download the broken file, perhaps via browser and not the downloader.
Let the installer run another integrity check when you re-try to install it then. If the problem persists it would be best to write to the support, there is not much we can do to help you here.