You get it by completing Domestic A (Gold) if you are with PAL but if you are with NA you get it by completing A License (Gold) but I don't know about JP
Actually that applies to the "GranTurismo Skyline" which is a more powerful R34 GT-R with the GT paint scheme. The actual pace car with the flashing lights - but unfortunately ordinary GT-R power figures - is the prize car of the Real Circuit Tours in the Extreme Hall.