Gasto was commander of the Rigellian XXV Imperial Guard Regiment, the main bulk of the forces of Cardinal Bucharis during the Plague of Unbelief. He met Bucharis on Rhanda and proceeded to conquer an empire for the Apostate Cardinal. His forces swept across hundreds of planets and were the main regiment to attack The Fang, although this failed. Most of his forces were killed when Sehalla and the Imperial Navy retreated but Gasto's own fate is unknown.
His forces were unable to take the planet Chiros, due to the massive uprising of the population, and even Gasto could not rouse sufficient forces to take the world. His men also rebelled against Bucharis when the end came, allowing the rioters into the Cardinal Palace.