It is a fairly simple conversion of the Smiffden Indycar mod to work in the GTP mod (I posted the instructions on NoGrip years ago); though there may be a newer Indycar mod by now, the procedure is the same. You will need a second install of GTP, unless you want to manually swap the series and sound folders every time you want to switch GTP and Indy; the series menu is disabled in the GTP mod so only works with the default series, which must be named with the GTP filenames. Beyond that it is the same as switching any carset to another physics type - extracting the series dat file, renaming some files, editing the individual car files so they are recognized with the physics change, and getting a good sound set. And you can use any of the four physics, they all seem quite appropriate for Indy or F1 (I have a '78 F1 set in my "Indy GTP").
If you don't want to try the conversion yourself I could upload the series folder somewhere, just move all the files from your series folder and replace with these; there are several "open wheel" sound sets around. And you'll get my mainback screen,lol