Gamma rays have such a high intensity, that makes them dangerous to humans. Gamma rays also have the ability to pass through ordinary objects, such as:
Gamma rays are also very dangerous because they have a very high radiation level, which can cause serious health issues such as cellular damage, and radiation sickness. It also causes mutations when it divides.
I feel like it should also be noted that this is how Bruce Banner turned himself into the Hulk. Use with caution.
Ironically, directed gamma rays are used in some forms of cancer treatment.
Gamma rays are used for cancer treatment because they damage cells. Since cancer is cells growing larger than they should be, tumors, gamma rays help kill these cells.