Was pretty sure this thread was a duplicate, but so far... nothing... though there have been similar threads.
I drive a lot in real life. I have a part-time gig testing cars and writing car reviews (partially the fault of GTPlanet for infatuating me with the idea) and as part of my work and family life, I've probably driven somewhere over 200,000 miles in the past two decades.
As part of that part-time gig, I have had the opportunity to drive quite a bit on the racetrack (but not as much as I'd like) and to experience a wide variety of cars both on the road and on the track.
For the most part, despite lacking some of the finer details like tire scrub, sidewall flex and that last 10% of lift-off oversteer, Gran Turismo is quite accurate. It understeers where you expect it to understeer, and it oversteers most of the time when you expect it to oversteer.
Some may disagree, and a lot of people complain about the understeer, but really, almost everything understeers on the racetrack, including cars with a reputation for oversteering. The problem is that without visceral feedback, it's often way too easy to misjudge entry speeds, and nothing contributes more to understeer than hurtling into a corner way too hot.
Sports Hard tires are certainly authentic for most sports cars, though some cars seem to have been miscategorized (upgraded to SH or downgraded to CS).
SH gives you a feel of what the car would do on your typical UHP Summer tire. Most of the aids won't, because they're too simplistic and general to replicate the wide variety and levels of traction and stability control available on the market today. And even then, on the racetrack, most aids simply suck.
Your results WILL vary in real life, though... and driving in GT5 may teach you the basics, but it won't teach you anything about "feel"... that comes through experience behind the wheel.
Good teacher, though. Learn the discipline it takes to do a fastest lap... slow in, fast out, throttle and brake modulation... stabilize the car under braking before turning, stabilize the car after the turn before accelerating... and you should do fine at the racetrack.