Belly Up is a seafood processing plant in Liberty City circa 1998. They use Mule trucks/vans (specifically, Triad Fish Vans, a.k.a. Belly in GTA Advance) that are seen driving around Portland Island in Grand Theft Auto III, Grand Theft Auto: Liberty City Stories and Grand Theft Auto Advance. The name "belly up", as well as the logo, is a take on dead fish.
Crates bearing the Belly Up logo can be seen at the docks in East Hook, Broker, Liberty City in Grand Theft Auto IV. They can easily be found right across the street from Brucie's Executive Auto Garage.