This new outfit will be unlocked after completing Colonel Cortez's "Treacherous Swine" mission, which can be found at the North Point Mall in the Gash store. The outfit consists of a gray t-shirt, covered by Claude's leather jacket from the third part of the GTA, as well as blue jeans and white sneakers.
Installation: Open gta3.img with the IMG Tool and replace the files, or use a modloader, creating a folder under it for modding.