I remember these exploits from all the online era GT games and never ever was anyone banned for using them, or had anything obtained illegitimately taken away by PD. At least not as far as I'm aware.
In GT5 you could create multiple users on your PS3 and enter a specific birthdate to get gift tickets for cars from the corresponding year (i. e. enter 3rd July 1970 as your birthdate to get a 1970 car ticket). If there were multiple cars from that year, you could simply reload your save file to reroll the dice until the ticket gave you the car you wanted.
There also was a glitch on Special Stage Route 7 where you could get out of bounds and finish a 200 lap race in just a few minutes. Since the prize money scaled with the length of the race (and the number of participants) there were tons of online lobbies with people exploiting it.
GT5 is also the only present iteration where it was possible for players to send cars to one another. This opened the opportunity for people to sell cars for real money, which was capitalized on especially by the few who had the skills to beat the Red Bull challenge early on and get the X2010 as a reward. Since they could just back up their save files before sending the car off, they could essentially sell it an infinite amount of times. I remember seeing ebay listing for 70 € a piece. This is likely the main reason for why microtransactions were added from GT6 onward.
I wasn't as active in GT6 as I was in GT5, but I do remember reading about people doing money/vehicle exploits in that game as well.
GT Sport had the Nürburgring pit lane glitch which allowed you to beat Gr.1 cars with an N100 Miata and claim huge money/exp rewards in the process, drastically shortening the time it took to get all cars and the Platinum Trophy.
I myself used glitches in GT5 and GTS and I'm happily making use of this one in GT7 as well, given the egregious ingame economy that is designed to drive players towards MTXs. The glitch has netted me about 50m at this point and it's very likely that I will be "set for life" by the time it's patched out. Even all those years ago there have been people advocating for banning players using these glitches, but the simple fact is that judging from the past it's extremely unlikely that that is ever going to happen. The only instance that I'm aware of where people got sanctioned was when players were using a glitch to shorten their pit time in online races in GT Sport. And that had nothing to do with getting money or cars, it was for giving yourself an unfair advantage in a competetive environment.
