Guncraft uses steam workshop to share user created content between players. This includes skins, prefabs, guns, melee weapons and maps. Currently there are over 4000 available items on Guncraft's workshop. All you need to do is click the subscribe button and wait for your item to be downloaded.
Items can be set to public or it can be set to friends only, thus allowing only your friends to use your item. It should be noted that nobody plays this game that the devs stopped caring about it after nobody bought it. Damn shame for the kids that grew up plaything this.