There is none. Much of what's said about it is people believing that, because the updates might not be to their taste, GT8 must be in development and PD must be saving up the good content that they want (somehow knowing precisely what the good content that they want is) for it.
It's largely a 50:50 mix of hopium and copium and rather misses two important details: that Yamauchi has said they avoid doing that now (this is rationalised away by highlighting other things Yamauchi has said that have either not come to pass or have subsequently changed); and the last time there was a new game in the groove and on its way, PD stopped updating the old one and the content saved up for the new game arrived in very roughly the same quantity it would have been had they just kept updating the old one monthly...
In practical terms, the next Gran Turismo game would have been in planning before GT7 launched. We don't know how advanced a state it would be, but I would suggest that, after three and a half years, it's likely to be fleshed-out in concept and may even have running pre-alpha versions, or something playable far from the final form. I'd also say it's likely that the next GT won't be radically different in terms of development than Sport/7, so it should be pretty rapid to finish off.
However, as previously noted, GT7 is still seeing decent player engagement from what we can see in public (Daily Races, Time Trials) and it's hovering around the top 20 PS5 sales - breaking into them when discounted. The bosses at PlayStation Studios/SIE will probably not want to pour money into a new game when the old one is still doing relatively well and not green-light it for final development.
Not only that, there may well be considerations over a PS6 version of the game. We know next-to nothing about the PS6, but it's likely that if it's on the slab for "Holiday" 2027 there should be dev kits for first-party studios like PD already. The way that the timings are lining up, I wouldn't be surprised if we see GTNext launch as a PS5/6 cross-gen with the PS6 launch - though no full GT game has launched with a PlayStation to date, so I also wouldn't be surprised if it doesn't.
As a fun data point, though we keep mentioning the lack of new tracks in our update articles and videos, and I think have done for six months now, GT7 is actually almost the best we've ever had it for cars per track (both locations and variations). Only GT Sport and GT5 Prologue were better, and 7 has the second-most locations (just behind GT6) and most variations of any title in the series.
