The online community of this game is probably as active as here on E2. If you do a web search on GPL, you'll likely find someone talking about car setups, race car physics, or driving technique.

Stock Tracks:

Kyalami, South Africa
Mexico City, Mexiuco
Watkins Glen, USA
Monza, Italy
Monte Carlo, Monaco
Spa-Francorchamps, Belgium
Rouen-les-Essarts, France
Mosport, Canada
Silverstone, Great Britain
Zandvoort, Holland
Nurburgring, Germany

Cars Available:

Lotus-Ford 49
Repco-Brabham BT24
BRM P115
Eagle-Weslake T1G
"Coventry" aka Cooper Climax
"Murasama" aka Honda
Ferrari 312

Things you can fiddle with on your car:

each wheel:
Tire Pressure
Wheel Rate (springs)
Bump Rubber

per axle:
Roll bar stiffness
Static Ride Height

Transmission stuff:
Transmission Ratios (per gear)
Final Drive Ratios (per gear)
Differential Ramp Angles (power and coast side)
Differential Ratio
Clutches (1-5)
Front Brake Bias
Fuel Level

The only data you get about the cars state is in the tire temps table. Inside, middle, and outside temps for each tire are displayed, while you're in the pits. From this you can change your camber settings, tire pressure, but the rest you're on your own figuring out. There are a number of websites and a 90 page book that comes with the game to help you.

I've played this game for almost 2 years and I still don't know what half of the settings do. I've done fairly well by sticking to the stock settings and doing a couple tweaks to the differential and gear ratios. Lots of more knowledgeable people (and some that aren't) trade car setups.

In addition to the stock tracks, there are about 200 netizen made tracks, several racing leagues, and even a site called GPLRank where you can upload your best times and get comparisons.

You can look at multiple camera angle replays and save them to disk. There is a program (GPLRA) that will extract telemetry from this, show you your racing line, plot your traction circle, graph speed and compare with other laps (your's or someone elses)

This is one of the hardest games around. The cars are modeled with such complexity (I can't, lacking personal experience driving vintage F1 cars, claim accuracy), that many people complain about how hard it is. Weight transfer and g-forces aren't well translated to a computer screen based game. Well, should games be easy? I think not. This very problem is a blessing for some, and explains the game's rabid following, even 4 years after its release.

A sequel has not been made, nor is one in planning (unless you believe net rumors. My theory is that Papy took their modeling engine, went to a more popular sport (in the US), NASCAR, and made bundles of sales from that.