I golded it first try last night (using G27). I agree it is one of the most difficult series in A Spec but not impossible. Very rewarding but totally exhausted and covered in sweat after 140 laps in the FGT!
Car setup is very important (as it is in real life F1!). Search for "Formula GT setup" on this forum and be prepared to battle for 3 hours (approx) to finish all races. Don't do it on a night when you're tired or tipsy, it requires huge amounts of concentration. One mistake and you end up losing my places (just like it is in f1!)
Some tips: I found that brake bias is paramountly important with FGTs, wrongly setup the car tends to get tail happy every single corner which can be taxing to drive 20 laps in. 3F 1R with ABS 1 is what I used most but the car is extremely powerful and twitchy in certain situations so you just have to be on your gaurd all the time. Very small room for error (just like it is in real f1!)
Set the suspension neutral front and back, the car is pretty balanced so no need to bias for understeer or oversteer, but here's one setup that I quickly searched for in case you're having trouble finding a setup:
downforce max/max
diff 15/20/15
ride height -5/0
spring rate 16.4/17.5
damper ext 6/8
damper comp 5/7
anti roll 5/7
camber -2.5/-1.5
toe -0.25/-0.15
trans 249 (or 400km/h)
brakes 4/1
I used max down force front and back for all races except Monza, but to be honest, even at Monza, max downforce would be OK.
Keep at it! Don't lose heart if you crash 2-3 times each race. I did this in nearly every race so effectively starting from 12th position, but so long as you have at least 5 laps left you can catch up and win. I did this 5 times!! very rewarding to catch and overtake with cars evenly matched. Many times I imagined I was Bob, decrease pace and take it easy so as not to make a mistake, increase pace to catch up and overtake lol
Best/Most challenging GT5 experience to date!