Gamma Halo, formally Installation 03, was one of seven Halos in the Neoteric Halo Array. At the tail end of the Forerunner-Flood War in 97,445 BCE, Gamma Halo was fired for the only time as part of the Great Purification that cleansed the galaxy of all sentient life and eliminated the Flood threat.
It is currently in an unknown location due to the monitor of the Composer’s Forge, 859 Static Carillon, transporting it via slipspace to make repairs after the Mission to Installation 03. The ring was originally discovered in 2552 after a freighter escaping the Battle of Tribute arrived at the installation by accident, and less than a year later the UNSC had set up an outpost nearby the halo known as Ivanoff Station.
In 2557, Gamma Halo became the focus of conflict again with the arrival of the Ur-Didact following the Battle of Requiem. Scientists on Ivanoff Station had recently extracted a Forerunner device known as the Composer from the surface of Gamma Halo, and were studying it when the Ur-Didact arrived to steal the Composer for use against humanity known as the New Phoenix Incident.
Immediately following the Ur-Didact’s attack on Earth, John-117 and Blue Team were sent back to Installation 03, where the Didact had been transported following the detonation of a nuclear device at the end of the New Phoenix Incident. There, the Didact killed all of Spartan-II Black Team, but Blue Team, with the help of Static Carillon, were able to use the Composer’s Forge to seemingly destroy the Didact.