From my personal point of view: Nothing much.
Driving in the streets is completely different. You shouldn't be driving with speed in mind, but with safety in mind. Driving is like a language, first you learn the words (i.e how to operate the car's controls) and then you learn the proper ettiquete on using them (i.e how you should bevahe on the road),. My main advice would be to stay sharp and take every factor in account. Don't take it lightly and don't get too creative: there's no restart button in real cars.
If we're talking about real-life racing, I'd say GT5, and other simulators as well, are definitevely helpful but in no way a guarantee of success. You might know what a late apex is while sitting in your comfy chair with you plastic Logitech wheel in your hands, but remembering what a late apex is while your ears are full of Vroom and your entire body is being hammered by gravity is an entirely different story. My suggestion is to move from GT5 to harsher simulators and then to a racing academy. If an instructor in an academy tells you that what you did in the game is wrong, then take his word for truth.
All in all, leave the GT5 mindset when you seat behind the wheel in a reall car. I'm also a learning driver and I know how hard it is to be a speedfreak confined to streets full of slowpokes and scum taxi drivers, but if there's something I learnt is what the Legendary Fangio used to say: A crazy man finishes in the cemetery. Seriously, drive like your life was at stake because it actually is, unlike in GT5 or even iRacing