Oh lawd teh misinformation.
GT1's showing of individual gear speeds was excellent. I *think* GT2 had the 25-position autoset and know for a fact GT3/4 did. GT5/6 use the exact same system (still 25 increments too) just with autoset rebranded as "top speed".
Ummm... You wouldn't be able to set the same gears doing so. You'd wind up with a completely different set of ratios available.
Example of how it actually works (arbitrary numbers but the math should be fairly correct)
If final is at, say, 2.500, you get [6.000/4.000/3.000/2.250/1.750/1.500] for x top speed setting. Move the final to 5.000, wiggle the top speed slider to reset the ratios to match the new final, and you'll get something close to [3.000/2.000/1.500/1.125/0.875/0.750]. Then when you move the final gear back down, HEY! Closely spaced gears, except long!
That's not how this works, that's not how any of this works. If you don't give how you set the trans up, nobody will be able to get the ratios you used. I've been known to use arbitrary base finals to set ratios as I generally use minimum final (there's a very minor gain to be had in acceleration from doing so while maintaining the same overall ratios).
Anyway, point being it isn't exactly a glitch. It's been baked in to Gran Turismo for at least two console generations, they wouldn't simply say "NOPE THIS ISN'T HOW THIS WORKS ANYMORE" without saying something about it. Therefore, the whole "yeah let's just default your gearing" thing going on with 1.17 is in fact the glitch, not gearing working how it was always intended to.