There is no 'official' community, and here is as good a place to ask as any,
but alot of gpl fans use
http://forum.racesimcentral.com/forumdisplay.php?s=&forumid=36
There are sections for each aspect of the sim, including tracks, car
updates, setups, videos & so on.
If you want to update the graphics as realistically as possible, and
assuming your PC can handle it, a good place to start is with the GPLEA cars
from http://gplea.racesimcentral.com/ although their Brabham hasn't been
released yet, so look for Bruce's Brabham (sorry, I don't have a working
link). These car updates need quite a recent graphics card & CPU to keep the
framerate up, particularly Bruce's latest Brab.
There are only 7 cars, so although you can make them look like different
models (eg Honda RA300 or RA273 but not both at the same time) but they will
physically feel the same as before. There are a few projects working on
altering the physical values of the cars (power, grip etc) but none that are
widely available yet.
After that, each original track has had various updates over the past few
years, some with complete packages available, such as 'Monaco Rocks' or 'a
small add-on for Monza'. You'll just have to dig around for these, but even
after 5 years the mods are still coming thick & fast, and if you can't find
something & know it should be there, make it ;-)
Malc.