Hate to break it to you, but in terms of online longevity PD and the GT series has a very poor track record.
iRacing's a poor comparison, if anything made Sony look at PC for the GT series it was the success FH4 had when it hit Steam (and despite having been out on XBox and on PC via the MS store for quite some time already).
Forza Horizon 4 finally made its way to Steam this week, and immediately shot to the top of the rankings for a title in the racing game genre. The game's popularity has never been in doubt. It rocketed past ten million players in its first nine months on sale, reaching 12m by the end of its first
www.gtplanet.net
It really, really isn't.
Its visuals, photomode, and replays are matched by a number of PC titles.
Its track and Car list is not as varied as other titles on Playstation let alone PC (numbers don't make for a balanced car and tracklist)
GT's career mode has also sucked for quite a while now, the brain-dead AI, with chase-the-rabbit AI needed to die a long time ago.
All of the above is further shown to not be as advantageous when you throw in titles that support mods. Quite frankly in terms of all of the above, Assetto Corsa, on PC, with mods, runs rings around GT. Hell, it's got GT titles that GTS doesn't have (and haven't been confirmed for GT& yet).
Not even close to a high-end graphic card (and that's 150km long road):
What GT has, and what would help it, is a solidly accessible balance of light-sim gameplay, if they can match that to a career mode that is actually worth a damn, then it a title that PC gamers will buy.