There are three splendid different addon versions of the Le Mans Circuit de la Sarthe (much more accurate and better than the "Le Mans 24 Hours" game version) for Sports Car GT (two of which, 1986 and 1989, are without chicanes). All of these tracks have both day and night versions, and wet versions of both (with really splendid falling rain about as good as GP3's {and actually emabarassingly much better than the "rain" in EA's F1 2001} now available as an addon!). Alas, the addon day-to-night transition for Sports Car GT tracks doesn't seem to do much of anything noticeable.
There are also a mess of addon cars for Sports Car GT, the real versions of which actually raced at the 24 Heures du Man races from 1955 to 2002.
Other Sports Car GT addons allow you to have races of up to 72 hours in duration.
Sports Car GT now also has no less than three addon versions of the Nurburgring Nordschleife (including the combined Nordschleife and Grand Prix 24 hours circuit, 15.721 miles in length), with yet a fourth being worked on.
As Sports Car GT can currently be purchased on e-Bay for about $5.00, I just thought that I would mention a few of this game's attractions that cannot be implemented in Grand Prix 3.