The Gamma frequency is a type of radio originally used by the Colonies hundreds of yahren prior to the Fall of the Twelve Colonies. It was capable of broadcasting audio and video signals, much like television signals.
According to Lt. Boomer, Gamma frequencies are no longer used by the Colonials because the signals bounce around and would continue to travel outward. Gamma frequencies can either be harmonic (a term used to describe signals limited to a short-distance) or extra-galactic.
The Colonials employed equipment for transmission and reception of Gamma frequencies. Captain Apollo discovers the Galactica contains such equipment its last remaining, outdated celestial chamber. Apollo is able to repair the components in the chamber, which are used to record a signal that he believes may have originated from Earth. After providing the transmission, which contains heavy interference, Boomer offers to create a Gamma frequency booster to enhance the quality of the signal reception.
Much to Apollo's disappointment, the only recording of the Gamma frequency transmission is destroyed in the battle with the Cylon basestar. It is unknown whether or not Apollo manages to record the clearer transmission -- which does appear to originate from Earth and is of the first moon landing (TOS: "The Hand of God").