The Gamma frequency was a radio-type signal 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 were no longer used by the Colonials because the singals would bounce around and would continue to travel outward. Gamma frequences could 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 Captain Apollo discovered The Battlestar Galactica contained such equipment in one of the ship's remaining, outdated celestial chambers. Apollo was able to repair the compontents in the chamber, which was used to record a signal that he believed originated from Earth. After providing the transmission, which contained heavy interference, Boomer offered to create a Gamma frequency booster to enhance the quality of the signal reception.
Much to Apollo's misfortune, the only recording of the Gamma frequency transmission was destroyed in the battle with the Cylon basestar. It is unknown whether or not Apollo managed to record the clearer transmission -- which did originate from Earth and was of the first moon landing (The Hand of God).