From the official website.
“As previously mentioned in past announcements, the remaining online services for Gran Turismo 5, including Online Lobby, Seasonal Events, Museum, News, Community, Remote Race, Log in Bonus, Online Dealership and DLC purchase, will no longer be available after 03:00 EDT, 30 May, 2014.”
“We would like to thank everyone for their passion in supporting Gran Turismo 5’s online services, and we look forward to providing even better online services for Gran Turismo 6.”
Here’s the trailer for the game, just for old time’s sake. Rest In Peace Gran Turismo 5!