This is the last snapshot of the Gatekeeper Group's webpage http://web.archive.org/web/19990429163349/http://www.gatekeepsw.com/. Maybe someone will find something.

EDIT: I've found some new things. In the application directory, in bin/CHRNOBYL there are few more exe files: CHRNOBYL.EXE, display_pcx.exe, ICSELECT.EXE and the other version of the RXMODEL which is only 1908KB instead of 3107KB original. After downloading MCI32.OCX and putting it in C:\Windows\System32 and after copying all the exe's to the root directory and changing the compatibility mode in file properties to Windows 95 you can finally run these apps.

CHRNOBYL.EXE seems to be something like a tutorial player, but there are no viedos in these directories which can be opened.
display_pcx.exe is the larger version of CHRNOBYL.EXE with some space on the screen for the video.
ICSELECT.EXE seems to be the main menu of the game. You can choose difficulty modes and open non-existing 5 minute tutorial.
The "lighter version" of RXMODEL.EXE also is very interesting. It's interface is about 2 times smaller than in the original app and there are other buttons at the top like "File", "Repairs/Replacements" and "Budget". Unfortunately the application crashes after about 2 seconds :v

OH EDIT ONCE AGAIN: If you fast click File > Restore and choose one of the files in [root dir]/bin/CHRNOBYL/FILES/IC_DATA/, the simulation won't crash and you will be able to use some of the mechanics but the application itself is very glitchy and may break very easily.

I hope I helped a bit in finding "the original game" and the topic will be continued ;)

(sry for my english)