This is not illegal. Please read here (rough translation from Russian) and you will see that a moderator on the official GSC Game World forums said it is ok to use STALKER resources as modifications for other games, non commercially, if proper credit is given.Credits:
Model/Textures: GSC Game World
Compile / Port: DeadPixel
If you had problems with the gun being silvery, please re-download.
I wish I could try it out but when I try and use the only mirrored download it says the network is taking to long and when I click mark as broken, it says its fine.
If there is another way to download this let me know or send me a pm please.