Gum or Periodontal disease in dogs is only reversible if you take care of the problems that lead to the gum or periodontal disease before it starts.
But sadly once your dog has got Periodontal Disease or any form of gum disease then the damage is already done and cannot be reversed.
You can treat the Periodontal Disease in the dog but there's no cure for it.
Treatment for the Periodontal Disease in your dog can cost from $2,000.00 to $3,000.00 but they can't do much other than cleaning and getting rid of some bacteria in the dogs mouth to prevent further damage.