"It is just an illusion, manufactured in a lab next to the sarcophagus..." The subject matter of this article was cut from the final version of a S.T.A.L.K.E.R. game.
The GSh-18 is a pistol that was cut from the final version of S.T.A.L.K.E.R.: Shadow of Chernobyl. The only remains of this weapon are a texture file (wpn_gsh18.dds) that can be found within the game files. Presumably this pistol would be chambered in 9x19mm just like its real life counterpart.