Here's why:

* Atomic number represents the number of protons in an atom's nucleus.

* Gamma decay is the emission of a gamma ray, which is a high-energy photon. Photons have no mass or charge, so they don't affect the number of protons in the nucleus.

What gamma decay does:

* Changes the energy state of the nucleus. An excited nucleus releases energy in the form of a gamma ray, transitioning to a lower energy state.

* Can occur after other radioactive decays. Often, alpha or beta decay leaves a nucleus in an excited state, which then undergoes gamma decay to reach a stable state.

In summary, gamma decay is a process that alters the energy of a nucleus but doesn't affect its composition, including the number of protons.