Gum disease can sometime be reversed as long as the gum disease is only mild and not severe.
If the gum disease is only mild it's known as gingivitis which can usually be reversed if caught soon enough and treated soon enough before the gum disease progresses.
Gingivitis is treated and reversed through daily teeth brushing and flossing, and regular cleaning by a dentist or dental hygienist.
If the gum disease is too far progressed then the only thing that can be done is prevent further progression of the gum disease.
The longer you leave the gum disease untreated the worse it will get and eventually the gum disease can lead to tooth decay and other health issues.