Sadly, the GP1 is one of the cars that were gimped by R* giving it a very low downforce value (1.5 I think, compare this to the 3 that the RE-7B has) making it understeer badly on turns.
Question: Is the GP1 worth getting?
Such a shame, because its one of the best designed cars in the game IMO.
The same issue affects the ItaliGBT Custom and Specter Custom (making them slower than their normal counterparts); the Ruston and the Comet Retro Custom (in a lesser degree).
edit: if you really like the car, get it, but don't expect it to be competitive in races unless R* "fixes" it (which may never happen).