Navagon: Not that. The fact that installers from elsewhere are being associated with the GOG downloader. Removing those associations should remove the problem.
If his AV is thinking that all installers are bad anyway then it's time to ditch that too.
Cleidophoros: Installer from elsewhere is NOT being associated with GOG downloader; Gamersgate installer is the same as GOG downloader; publisher for the game provivded the same package to both sides. When the installer pops up with GOG logo on it he thought it was being associated with GOG downloader.
gamersgate installer is the same as gog installer* the gog downloader is a completely different program entirely.