I think you can easily come to basic approach to setting up the majority of GPL cars (F1 type cars anyway) by just looking at a collection of setups. Here's an example of a setup (in metric units) that would work for most cars, ranging from 65 to 67:
Front Tyre Pressure: 131 to 159 for F1 cars, 152-172 for F2 mod.
Front Springs: 140
Front Bump: 2
Front Rebound: 2 or 3
Front Camber: -0.50 to -1.25
Front Bump Rubber: almost always minimum
Front Ride Height: almost always minimum
Front Toe: -0.064 to -0.508 (less with 65, 66 2L and F2, more, in negative direction, with 66 3L and 67)
Rear Toe: 0.127 to 0.699 (same as above, except in positive direction)
Rear Tyre Pressure: same as Front
Rear Spring: 193
Rear Bump: 2 or 3
Rear Rebound: 3
Rear Camber: -0.25 to -0.75
Rear Bump Rubber: almost always minimum
Rear Ride Height: almost always minimum
Pressures and Camber should be set according to the tyre temps reading after a stint on the track. Try to equalize all temps across the thread. The only exception is on fast tracks, where higher pressures give more speed down the straight, due to less roll resistance.
As for drivetrain settings, set gears so you keep the car in maximum torque range in the middle of the corner. The differential is really a matter of personal preference, so you only need to understand what each of the settings of the diff does to set them accordingly, to experiment. I prefer more lock on power, less lock on coast, and 1 or 2 clutches at most. I usually start with 45/60/1 or 60/60/1-2, regardless of mod.
Some cars (like 67 BRM or 65 Honda) are an exception of this setup base, due to their different weight distribution, but most of the cars work fine with that.
69 are a bit different because of the wings, so they change more than no-wings cars from track to track.
Now, if you want to know what to tweak to fine tune the car by following some simple guidelines without actually understanding the parameters, there are plenty such online. I'm not a fan of those, as I said, because they're never quite correct for all situations or all cars. Here's one though:
http://www3.sympatico.ca/cbarnett/SetupMatrix.html
