Gamma radiation

 was the name of a type of radiation that could originate in a solar flare and be collected by a collector which included components of Dalekanium. The Daleks called the expected surge of gamma radiation a gamma strike. When the solar flare and the surge reached the Earth, it travelled to the collector on the night side and was either concurrent with or took the form of a bolt of lightning. (TV: Evolution of the Daleks)

According to another account, gamma radiation was released during nuclear fission, when an atom split into two after absorbing a neutron. (AUDIO: Power Play)

President Rassilon condemned General Trave to be executed by having his mind and body obliterated in public by gamma ray. (AUDIO: Havoc)

The Seventh Doctor noted that the destruction of a passing strange matter asteroid would send off a blast of gamma rays equivalent to a supernova, incinerating the planet Lakertya and everything else in its corner of the galaxy. (TV: Time and the Rani)

Behind the scenes
Either there was a second type of energy released by solar flares with the same name, or gamma radiation behaved differently in the DWU. In the real world, gamma radiation was very similar to the type of roentgen radiation used in X-ray machines, the type of radiation amplified and used on the Slab in Smith and Jones. However, these types of radiation were invisible and travelled in a straight line, were largely absorbed by the atmosphere, and could be blocked by a significant layer of rocks and metals. The gamma radiation from a solar flare at night would be blocked by the bulk of the earth and the atmosphere, and would not manifest as a bolt of lightning.