This is called a "mod". You can make them by ripping 3D models from the original game and inserting them into others with Garry's Mod. The 3D models can be objects or serve as "skins" for already existing objects, NPC's or even your character.
https://www.youtube.com/watch?v=yl8UcULuw7A&t=1m20sYou can also use them to make animations with the freeware MMD (MikuMikuDance) program.
https://www.youtube.com/watch?v=yyO41aHhGq0