The one thing PC does well is cater for the more niche markets, the simulators, and that would be your Assetto Corsa, iRacing, rFactor and so on.
If thats not your thing, then you're looking at casual titles, which is exactly what Gran Turismo & Forza are (however they chose to portray themselves), and as they're not niche market, they're titles which tend to be big sellers because they appeal to casual gamers with their attitude to realism, AI, and in some cases novelty aspects.
Theres not much for the PC which isnt multi-platform in that respect, because casual = lucrative.
The only casual exclusive i can think of would be 'Next Car Game' which has some of the realism of a simulator, but its effectively demolition derby rules, so its going to appeal to a more arcade audience and attitude. Should be a lot of fun when its finished, but its in Early Access on steam already
http://store.steampowered.com/app/228380 (wait for another sale).
After that, for the multi-platform titles you've got Grid Autosport which is out now, and Project CARS which is as much simulator as the SHIFT series (same developers) which is out at the end of the year (Nov?). Thats going to be in the same sort of space as GT and Forza IMO, though its been a few months since i tried it, but its that same casual simulation audience, even if it thinks its more highbrow its too dumbed down, but it should be a lot of fun in that Forza/GT market space, great graphics, fantastic weather effects, decent range of cars & tracks (see SHIFT) and just general fun for racing.
As i say though, there isnt an exclusive like GT/Forza which you've been missing out on, the closest we get is the multi-platforms from Codemasters & EA really.
