It's a good question. It depends on what the speaker is trying to convey. For example, if I saw my friend John at work, and he bought a new car, then when I get home, I would say to my wife (if I had a wife) "John said he HAS a new car" because the "HAVING" is still relevant, true information. If, however, I saw as I was leaving work, that John actually bought scooter. I would say to my gorgeous wife. ``John said he had a new car. What a liar``. The short answer: If you don`t know more about the context EITHER WAY IS FINE!

Gordon said he has a new car.

Gordon said he has a new car. "have" has to be in present tense because, as you stated, Gordon still has the car in question.

The rules of reported speech say to change both verbs: "Gordon said that he had a new car." You'd use this to mean 'the information which was true at the time of the report'. Whether or not it changed later is a moot point. If you want to indicate that Gordon's comment is still definitely true, then you can write "Gordon said that he has a new car." This is bending the rules somewhat, and this probably isn't a good example to illustrate the point.