I'm honestly not sure if they could or could not, but it almost certainly just would not work the same in GT as Iracing. This is because, as stated elsewhere in this thread, Iracing has a barrier of entry that means that pretty much everyone who plays it is a relatively hard core enthusiast. GT is played by millions, of all ages, shapes and sizes.
If they just copy-pasted Iracing's system, the game would be a wasteland over night. The races that actually happened would probably be awesome for the people involved, but unfortunately that is only one of the reason PD makes games. All of them need to be fulfilled. That includes making money.
I didn't mean as a copy/paste job as it's not a perfect system (far from it) but from what I've seen with the risk of damage and DQ's, it goes a long way to forcing certain people into self policing themselves better. Something GTS desperately needs.
To be honest, I hope they get tougher with the DQ's. Reach a certain level of contact in one race, you get DQ'd and sent to the naughty step for the next 3-5 race slots. DQ again that day gets you an even longer suspension. Make it painful and people will soon learn or at least try to race better.
If PD are serious about getting into esport, they need to stop chasing their tail trying to find an automated penalty system that simply doesn't exist. There will always be accidents and incidents that confuse it along with people who know how to game it.
It's the underlined bit that is making it work, not the existence of full damage. If you have full damage but still have "if the game lets you do it, it's legal", then people just hit other cars to cause them to have a head on collision with a barrier or tyre wall, ending the race of their victim while they carry on just fine.
Too cynical by far as the unscrupulous drivers are few and far between.
I've met a few of them but I'm not daft enough to gift them that opportunity. Not if my car and race is at risk.