Hi, last week I had the same frustration as the OP about these challenges, as I already knew they were going to be tough because I saw my brother´s own experience before I did it, it´s his game disk and after I borrowed it. So I´ll tell mine and his experience about this. I am a pretty decent player with the pad, and I have own every single thing since the first GT with it, so when I faced the Vettel challenge I tought I could make it. And also I wouldn´t glitch a game, that´s cheating...
The first I did was Monza, took about 45 min to learn how to use the car and to gold it with the DS3 as it´s the easiest of them because you can cut the chicanes and when you do that properly it´s a piece of cake.
Then went up for the Nurburgring GP, that´s hard.. I was able to get silver after like 10 or 11 hours. But I knew that would take many time to do 2 perfect laps, as for my brother he golded it in 3 days straight only stop for eating, bath and some sleep. So I knew it would be doable but I didn´t have that much time disposal, I got an old wheel I had at home with the brake pedal broken (glued it with superglue or something), and when the car goes in a straight line the wheel is not in the center is almost 60 degreess, but in about less then 3 hours did it with a wheel is way easier to do it because it´s smoother then with the ds3.
I tried hard suzuka also with the Ds3 for several hours but only got silver and my brother also got only silver in this one as we couldn´t make 2 perfect laps, so I did it for me and him with the wheel. The first S turns are hard to do with the Ds3.
So after this I know it is DOABLE with the DS3 but it would take so much time and effort but I recognize that for a player with medium skill would take much more time to do and also patience, and many people won´t be able to do it since the lower skill for some people even with practice won´t be enough.
This thread was very helpful for me for reference times and stuff,
https://www.gtplanet.net/forum/showthread.php?t=154480
Also some people wanted tips to do it, when I used with the DS3 and with much more time and practice I know the gold would come don´t give up, used the sensitive thing to 7 as the car turns faster, skid recov on, abs 1, the rest off, the driving line on is good so you are able to know when to brake but don´t use it as the game say.. take a look at the videos on the other thread, one thing that really helped me was when I knew my reference times were good and my line was almost perfect I turned everything on HUD off, just the car on the track so you can concentrate just in driving and won´t loose attention when looking for the split times if theyare OK or not. Just practice and you can make it, this is why challenges are for.. by far it´s the hardest of the game I know and it sucks you have to gold it for the platinum trophy but it´s part of the game.