That day is, of course, today, so as of now you'll no longer be able to access any of Gran Turismo 6's online features such as Open Lobby, Seasonal Events, Quick Match, and Community. Of course, the single-player portion of Gran Turismo 6 will remain playable.
Polyphony has provided some details regarding how this affects various aspects of the racer which you can see below. Let us know if you still had unfinished business with Gran Turismo 6.
Warning Regarding In-Game Credits
If you have deleted your game data after purchasing credits from the GT Store, please make sure to install/redeem the credits again before the end of the online service to have them available in-game. The installation/redemption of credits can be done in-game from [My Home] -> [GT Store] -> [Purchased Content]. It will no longer be possible to install/redeem credits after the end of the online service.
How to re-download Credits when your game data has been deleted
Even if you accidentally delete your game data after the end of the online service, it will still be possible to re-download and re-install the credits. To re-download them, go to the [Download List] in the PlayStation®Store, or go to [PSN] -> [Account Management] -> [Purchase History] -> [Download List] from the PlayStation®3 XMB (Cross Media Bar).
Regarding the “Track Path Editor”
With the end of the online services, the distribution of the “GT6 Track Path Editor”, currently available on “Google Play” and the “App Store”, will also end as of 28 March 2018. After the end of the online services, it will no longer be possible to transfer track data created with the “GT6 Track Path Editor” to “Gran Turismo 6”.