Rinse with dishwashing soap and water. If you don't want to do that, baking soda will help. It may turn peripheral grass brown, but if you're not worried about that, or the pea gravel takes up the entire thing, just use the baking soda.
Vinegar neutralizes urine odor really well, but it may be more work to rinse the vinegar blah blah blah to make sure it doesn't smell like pickled poo.
Soap and/or baking soda should be good. You can even sprinkle the baking soda and put fresh pea gravel on top. That would be nice to see fresh, new gravel for new homeowners. The baking soda would also be nontoxic to future dogs.
Cedar shavings are really awesome at absorbing odors too.
