first you need to aerate the soil. that can be alot of work depending on how much area you have. and what kind of substrate it is. add the gravel after. but the dogs may change their preference to another area. if they still like the dirt/gravel, then you can water the area every week to move the urine down into the soil.
also aerate the lawn. add some lime. water to push it down. and water often.
~ jo and Gotti Boi (aka Thunder Thighs, aka Jughead, aka The Doo Doo Monster, aka Sexual Chocolate, aka Mister Smiley, aka Sofa King)