Gamma rays are the most energetic form of electromagnetic radiation, with a very short wavelength of less than one-tenth of a nano-meter.
Matthew Bauerle writes in his article Physics Undergrad that, Gamma rays cause damage at a cellular level (mutations on DNA) and are penetrating, causing diffuse damage throughout the body. If the dose doesn't kill you outright the mutated DNA may make some cells cancerous. However, they are less ionising than alpha or beta particles, which are less penetrating.
According to BBC article about Tumour treatment, Gamma rays are strong enough to kill cancer cells from human body. This avoids the need of surgery and saves a lot of resources and doesn't leave an outside mark to the patient. Scientist are still studying gamma radiation and its potential side effect is damage to surrounding healthy tissue.