As others have said before me, the two best things you can do to improve your lap times are:
A) Use a racing wheel
B) Practice
When I first was exposed to the Green Hell in PGR2, I thought to myself "I will never be able to memorize braking points, entry speeds, etc. for the whole track." But after many hours on GT4, with a DualShock 2 never the less, I got a hang of it.
Now on GT5, with a wheel it is awe inspiring how much detail and feel there is of the road surface in my G25, and those many hours practicing the Ring are truly paying off.
I won't the first to say this but... it's hard to get Gold on these AMG challenges, but many of us on this forum will a test that its not impossible. It took me roughly one whole day to do the Beginner. Still working on Intermediate.
Be patient with it, experiment with different driving lines (turn off the aid), and above all else... practice.