In addition to what others have said, it also depends on the vehicle. The
defaults for the Lotus, Eagle, and Ferrari are pretty good, but the defaults
for the others vary from inadequate to horrible. The Honda and Brabham in
particular can be made a whole lot easier to drive if you set them up
properly.
Also there are simple tricks that you can do with the defaults that can take
as much as a second or two off of your lap times. Try some of these things:
1) Take out fuel to the minimum that you need. You need about 3 laps worth
for qualifying. Even short races require significantly less than the
default amount of fuel, allowing you to save hundreds of pounds.
2) Most of the default setups have the bump ***s way too high, causing
you to hit them during normal handling maneuvers. Bump ***s should only
be hit when going over bumps or landing from a jump, never during cornering
or braking. You need to have the bump ***s be about 2 inches shorter
than the ride height, more for the heavier cars or when carrying a lot of
fuel. You can lower the bump ***s to 0.5 inches using GPL Race Engineer
from Alison Hine's site, otherwise you will have to raise the ride height to
get the necessary clearance.
3) The tires in GPL seem to give the best grip when the temperature profile
across the tread surface is absolutely flat, even if this means they are at
a less than ideal temperature. So adjust the tire pressure and camber to
give you flat temperature profiles. In most cases this will mean setting
the tire pressure to about 20 psi cold and setting the camber
assymetrically.
4) The brake bias is arbitrarily set a bit high, probably to reduce spinning
for people who aren't used to the brakes. You can usually knock it back one
of two clicks without difficulty, more if you are a very smooth braker.
This will allow you to reduce your braking distance a bit and to brake
deeper into the corner entry. Small difference in lap times, but not hard
to do with practice.
With a Lotus or an Eagle and those four tweaks to the defaults, I can run in
the low 1:29's at Monza, less than half a second slower than my best. With
the defaults unchanged except for fuel, the best I can do is about 1:30.5.
Hope that helps.