Gran Turismo HD was initially intended as a limited time Christmas present of sorts for early PlayStation 3 adopters, but some nine months later, the free download is still available via the PlayStation Store. However, with Gran Turismo 5: Prologue on the horizon, Gran Turismo HD's days appear to be numbered -- at least in Japan.
http://blog.wired.com/games/2007/09/in-japan-gran-t.html reports that Sony will remove the title from the PlayStation Store in Japan this Sunday, September 30th. No word yet on whether the North American version of Gran Turismo HD will suffer the same fate, but we recommend downloading it immediately (wherever you are) if you haven't already. Along with the removal, the Japanese Gran Turismo website will no longer track rankings for the title, though the in-game leaderboards will continue to function.