Janet, welcome to TP. There is lots of support, experience, and good advice here in current threads as well as older ones.
(Helen, I saw your thread and will respond to you in there)
I am sorry to hear of the situation with your father and driving. If he is not safe to drive, then you are absolutely doing the right thing, to make sure he does not. Good for you. If that means you have the GP notify the DVLA, or do it yourself, then that is what it takes. I can hear you're worried about him and of course nobody would want an accident to happen.
Let me please say to you, as gently as possible, that likely nobody is going to be able to "convince" your father that he shouldn't be driving. This is not for lack of caring or trying or saying the right thing, but is the fault of the dementia.
Having said that, some approaches may work better than others.
In one of my dementia workshops I was told that when you have to deliver a message the person isn't going to want to hear (and this goes for people without dementia as well), that you have to consider not just the message, but also the messenger. They said that adult children were almost always the wrong messenger. It's much better to have some figure of authority, such as an agency (the DVLA), doctor, social worker, nurse, police officer, et cetera, or a peer, such as a friend/neighbor/adult sibling, or possibly a favored grandchild or niece.
With dementia, I find the authority figure route is usually most effective.
So you might say, I know you're upset about not driving right now, Dad; however, the DVLA/insurance company/doctor says you can't. Then use distraction or redirection, if possible, to change the subject.
I would combine this with some subterfuge as necessary, or if the GP is understanding, get them to deliver the message/write a letter/whatever you think will work.
I would also physically remove the car, so it's not possible that it can be driven. You can say it's gone for inspection or repairs or something. Again, come up with a kind but clear statement and just repeat it over and over. Dad, I know you want your car; it's having work done right now. I don't know how long it will take to get it fixed. I will find out for you. And then distract/redirect the conversation. (My mother is easily distracted with food, coffee and biscuits or ice cream being sure winners.)
If you can't physically remove the car right now, at least disable it and secure all the keys, but I think it's really best to get it out of sight. Otherwise you have a constant visual reminder.
Also please be reassured that if there is upset, it may not last forever. My own mother was livid about losing her car when she moved into the care home, but the angry comments about it soon stopped. She still refers to it from time to time (and often says she needs to go car shopping) but is no longer angry about it and is easily distracted.
Best of luck and I am sure others here will have some experience or advice to share.