Because they have no mass (at rest state), little mass (at velocity), and no charge. As a result, their interaction with matter is minor, thus their deep penetrating ability. Unfortunately, that...
Gamma rays are the most deadly type of rays. They are high in radio-active matter and can severely affect your body. Gamma rays are useful though. They can be sensed by military sattilites to find...
A gamma ray is an electromagnetic ray, and that is how they are related. Electromagnetic rays come in a range (a spectrum) of frequencies. We could also say that they come in a range of wavelengths...