It seems that Sony will be debuting Gran Turismo 6 on the 15th May, as the publisher will be introducing it in their 15th anniversary celebration for the IP.
According to the invite, Sony states, "Join us to celebrate 15 years of Gran Turismo, and discover exciting announcements about its future." The event will be at the Silverstone Circuit in Northamptonshire.
Sony also released a teaser trailer for the event. Check it out in the video below.