if gamma radiation causes cancer then why is it used to cure it?

Radiation damages cells. Depending on the dose, the cells get more and more damage until they die. A less than lethal dose of radiation leaves the cells damaged. Damaged cells may turn into cancer cells.

Cancer is generally treated in 2 ways:

Surgery: Remove the cancer cells (If surgeon misses one, the cancer comes back. Plus, in the process of doing the surgery, the cancer cells may be released to spread.

Kill the cancer cells in the body:

Chemotherapy: Give the patient very toxic chemicals (poisons) in the hope all the cancer cells are killed before the patient dies. This is why chemo patients get very sick and lose their hair. Miss killing one cancer cell, and it comes back stronger.

Radiation (Your question): Hit the cancer with enough radiation to kill it in the hope that you do not also kill or damage the patient

Gamma radiation dosen’t really cause cancer. Very few people get cancer this way. But the radiation is used to kill the cancer cells. And it works to the degree that it can slow down or stop cancer but is not always effective.