This is the Triad Gang car. It is one of the two Nissan cars in the game but the Skyline is a sports coupe. It has a big engine so it can get to a really fast speed quite quickly. It is not, however faster than the other sport motors in the game.
Although this is the Triad Gang car, it is also a hidden vehicle. You can find this car parked somewhere in Hyde Park in Free Roam Mode, next to the Lotus and TVR.
NOTE: The Triads have two gang cars including this one. At the beginning of the game, the Triads seem to drive Honda Civics. But, towards the latter stages of the game, they drive the Nissan Skyline.