Well. I've spent more time on real tracks than in any game or simulator combined. I can tell you this. Those so-called simulators (i.e. rFactor Pro, Cruden, or similar) are not really that close to the real thing, not to mention things like Assetto Corsa, iRacing, rFactor 2, etc. They have some similarities here and there (especially the replication of tracks), but that's about it. The very act of driving is quite different from real life track driving.
Track driving could be essentially summed up as this; To get faster lap time, you're reacting to the information fed to you. In real life, the butt feeling (the g force sensor) is the second most important source of information, well behind the most important source, visual, but also well ahead of other sources, like vibration, steering/pedal feeling, sound. So it's like 60% visual, 30% butt feeling, 5% steering, 4% pedal, 1% sound. But when you're on a simulator, you have either zero butt feeling or completely wrong butt feeling created by motion simulator (like D-Box, VRX, etc). It also lacks vibration, pedal feeling as well (by having either nothing or completely wrong simulation). As such, driving on a sim is like reacting to 95% visual, 4% steering and 1% sound, which is completely different from real life driving. Furthermore, those steering feeling and the behavior of car (to your input) are quite off as well.
This is the reason why top tier sim racing drivers don't perform very well on real tracks on their first try (and vice versa). They do better than regular newbie, but that's about it (i.e. nowhere close to serious/competitive amateurs or pro drivers). IMO, if you want to get a fast enough lap time on certain track and car (within 1% from the best possible lap for the same car/track), you typically need some basic skills AND some experience on that particular track with that particular car (around 100 laps). If you already have the required basic skills, then having a lot of experience on sim racing could cut down the amount of required laps to 70%, which is not much of gain (30 laps).
However, if you don't have the basic skill already, you need some experience/time to build that skill. That's usually some thousand laps (if you're a fast learner) or some tens of thousand laps (if you're not) for each tier of cars (roughly in this order; slow FF, slow FR, fast FR/MR, GT3; and each next level requires the basics from all lower level). Having a lot of experience on sim racing does not speed up this process dramatically (so you can't expect -99.9% or -99%, or even -90% time reduction). Although it indeed cut down some amount of required laps (something like -30%, -50%), but you still need thousands not hundreds. This is still a big enough hurdle for most people, because you need to spend roughly $300 for each hour of seat time when you're driving a cheap/slow FF car, and each time you progress to the next level, the price would go up three fold (mind that you need thousands of hours of experience overall just for the basics required for entry level race cars).
Anyway, you could divide racing games into two types. One of them could provide you those time reduction when you got a lot of experience on them (i.e. most sim racings), and the other one doesn't give you those time reduction no matter how much experience you have (i.e. from Mario Kart to Forza Horizon). Gran Turismo belongs to the former camp, and I don't think there is a meaningful difference among the games belong to the former camp, when it comes to the degree of time reduction.