I actually haven't 'completed' GT6, and I don't plan to either, the game hasn't shown me its worth the effort.
I've done everything (literally) but any of the S-class "endurance" races, as from what I've researched, they're just rigged weather-cycles with moronic A.I., surrounded by too many PS2/GT4 assets (in my eye), running on out-dated "physics", be it the tire/fuel wear or questionable aero/CoG.
I'd rather spend my time with GT6 tweaking race-cars and tuning project-cars, then driving them online with the few good drivers I've run into at the Nordschleife, filling time until I'm able to move-on to P.CARS and/or other PS4 games.