The Gash Blimp is a scenery blimp in Grand Theft Auto: Vice City. The blimp is owned by the Gash clothing store. It is non accessible to the player and can be modified only by third party trainers. The Blimp can be seen flying over Downtown Vice City, as an altitude far above the game's height limit (which makes it unreachable without modifications). If the player somehow manages to reach it, they will soon find that it's not solid at all, so they will pass right through it. It's similar in this way to the Airtrain and the DeadDodo (both of them unobtainable)
It can't be spawned via mods because it's not defined as a vehicle in the game files, instead, it's a moving object.