I still don't think Sony is stupid enough to do this, but looking at Polyphony Digital's Previous Game releases (not including localisation time as the games are basically done once they hit Japan):
Gran Turismo (PS1): December 1997
Gran Turismo 2 (PS1): December 1999
Gran Turismo 3 (PS2): April 2001
Gran Turismo 4 Prologue (PS2): December 2003
Gran Turismo 4 (PS2): December 2004
Gran Turismo 5 Prologue (PS3): December 2007
Gran Turismo PSP: October 2009
Gran Turismo 5 (PS3): November 2010
It's will of been roughly 3 years since Gran Turismo this Christmas, so I think it's plausible that GT6 could be ready to ship, but I think Sony would be much wiser just waiting an extra 12 months to up port the game to PS4.
However, if this rumour was right and Sony released GT6 this Christmas, there is potentially a FIVE year wait for the next mainline Gran Turismo (judging from the gaps between GT4->GT5. It would be foolish to let Sony's biggest selling franchise to arrive so late in a generation... AGAIN.