Gallivanter means "someone who roams around in search of amusement". This refers to Land Rover vehicles which frequently used as recreational off-road vehicle and Land Rover's reputation for producing excellent off-road vehicles (even the luxurious Range Rover is a good off-roader).
A Gallivanter building exists in Rockford Hills. It is not enterable and no Gallivanter cars are found inside of it.
It's possible that Gallivanter may be a subsidiary of Vapid since they manufacture the Huntley Sport, another Land Rover Range Rover inspired vehicle in GTA IV. On the other hand it may be a subsidiary of Ocelot since in real life Land Rover is a subsidiary of Jaguar. Another possible explaination is that Vapid sold the Gallivanter marque in 2008 (after the events of GTA IV) the same way in real life Ford sold both Land Rover and Jaguar Cars to the Indian Tata Motors, after which point the vehicles began being sold under the Gallivanter name rather than that of its parent company.