Yes, gamma rays can be harmful. The U.S. Environmental Protection Agency describes gamma rays as a form of ionizing radiation made of high-energy photons that can pass completely through the human body, causing ionizations that damage tissue and DNA along the way. Because of this, the EPA calls gamma rays a radiation hazard for the entire body.
Gamma rays are similar to visible light and X-rays but carry much more energy, and they often come from radioactive decay inside an atom’s nucleus. They are so penetrating that several inches of a dense material like lead, or a few feet of concrete, may be needed to stop them, which is why shielding is central to protecting people who work with gamma sources.
Source: U.S. EPA: Radiation Basics
Image: Radiation warning symbol (ISO 361), public domain, via Wikimedia Commons.