Source: http://ie.lbl.gov/education/glossary/AnimatedDecays/GammaDecay.html

In gamma decay excess energy is released from an unstable nucleus without a change in mass number or atomic number.

In the example shown above an isotope of element 66, Dysprosium, emits two gamma rays. Because this emission consists of only electromagnetic radiation and not a particle, the product is the same (albeit in a lower energy state).

The process can be represented in a balanced nuclear reaction using the isotopic symbols:

Clearly such a "reaction" is not very informative and it is unusual to write one.

In the example of U-238 alpha decay seen earlier a gamma ray is also emitted along with the alpha particle. That process could be represented like this:

Statistically, 77% of the Th-234 nuclei produced by this decay are in the ground state. 23% are in a higher energy state and will eventually emit a gamma ray of a specific wavelength (or energy):