Explain mass defect in terms of atomic mass number and atomic mass and explain how gamma radiation is produced.

The atomic nucleus is made up of neutrons and protons. If you measure its mass, it turns out that, it is always smaller than the sum of individuals masses of protons and neutrons that it is made of.
We call this difference a nuclear binding energy. This energy is used to hold the nucleus together. According to Einstein's famous formula:

Mass and energy are the same thing, or rather, a manifestation of the same thing. They are interchangeable. 
Some of the nucleus mass is used as energy to keep it together:

Gamma radiation is produced during gamma decay. 
When a nucleus undergoes alpha or beta decay it can be left excited. 
An excited nucleus is unstable, it has an excess energy. A nucleus that is excited emits a photon to get rid of this excess energy and we call this process gamma decay. During a gamma decay, no particles are eject from the nucleus. Gamma rays are simply photons that have very high energies.