Many oral surgeons are involved in hospital-based dentistry and the current field of oral surgery overlaps somewhat with plastic surgeons and ear,nose,throat physicians. They do gum surgeries and related procedures. Listen to the TV for a few minutes and you will hear about periodontal disease. It is a big problem. Many periodontists do tissue grafts where a tissue will be moved to replace tissue that has receded away from the tooth.
Periodontists can place implants as well.
This specialty is ever-growing with an aging population. Much of their time focuses on full mouth reconstructions of a person's bite. Their cases are usually highly complex. Many people think they are just there to make you look better, but that is just part of the story.