since it is a gravel driveway, pure clorox bleach will do the trick. bleach eats away at any bacteria also and kills anything. It even will kill the parvo virus. I recommend clorox for dog smell clean up.
Bleach and water 1/2 and 1/2 or Vinegar and water 1/2 and 1/2. Don’t use together as it will cause a chemical reaction. You can also pick up some bitter apple spray at the pet store which you can spray down later in all the spots you don’t want your pet to go.
first try weakening it by spraying it down with a hose. Then when dried, you can use Arm and Hammer Baking soda over the areas of concern. Try taking him for a couple of little walks during the day so he won’t pee so much on the gravel. He’d be happy and you’d be happy…. it’s a win win situation