@vkhibkin like Mitch said there is some discernible contextual differences. It all really depends on your experiences. Similar to Mitch i began drifting in 2012 in reality. I've driven a variety of cars since then from long wheelbased station wagons to little boxes like AE71 Corolla's. If you have enough seat time IRL the lacking sensations of Tyre slipping and weight transfers can be made up for by natural instinct and intuition. Especially if you have a G27 and have decent senses you will tend to pick up on these. So once i began to drift on GT6, these skills of mine that i've acquired translated really well, meaning i was drifting properly in a matter of minutes too. Personally i've noticed that GT6 is mostly realistic when you are driving cars under the 350hp mark. So yes Gran Turismo manages to simulate the experience of drifting well in reality.
Personally, i've used Gran Turismo as practice platform. Trying new methods of entering a corner and such. As Gran Turismo will really teach you the fundamentals of drifting which is throttle modulation, catching the oversteer, and countersteering.
I've also taught people to drift on Gran Turismo before trying it in reality. Contrary to
@tori said, these people have had no issue drifting in real life. Infact the only thing that hindered their IRL drifting was their car and setup of the vehicle. But their initiation and countersteering of the car was fine meaning they could easily handle the oversteer and manage keep a controlled slide.
HOWEVER, you have to be mindful that you can't drift as aggressively as you do in GT6, ensure you ease into it all. So that means slow entries into corners and such, especially if its your first go IRL.
Every account of drifting in GT6 i've mentioned has involved the use of ONLY a G27 and no DS3.
Here's some footage of me drifting back in late 2013. Happened to fall out of boost towards the end and the car gripped up severely tossing me around quite violently lol
Better drifting, proper setup, horrible video quality and filming lol
So yeah, in summary.
GT6 will help you get the fundamentals, and depending on the individual will train you in some aspects better than reality. Due to lack of sensation to some IRL aspects. Proving that GT6 sits at around 7.5-8.5/10 for simulation in my opinion anyway.