Short answers, pulled from the story.
Gallienus was a Roman emperor who ruled jointly with his father Valerian from 253 to 260, then alone until his assassination in September 268. His full name was Publius Licinius Egnatius Gallienus, and he was born around 218. His fifteen-year reign was the longest in half a century.
Gallienus became sole emperor after his father Valerian was captured by the Sassanid ruler Shapur I at the Battle of Edessa in 260. No Roman emperor had previously been taken prisoner by a foreign power. Control of the entire empire passed to Gallienus from that point.
The Gallic Empire was a breakaway Roman state that controlled Britain, Spain, much of Gaul, and parts of Germania. It was founded by the general Postumus in September 260, after he killed Gallienus's son Saloninus at Cologne and proclaimed himself emperor. The Gallic Empire remained independent until 274, outlasting Gallienus by six years.
Gallienus was the first Roman emperor to commission primarily cavalry units, called the Comitatenses, as a mobile strategic reserve deployable across the empire. He also barred senators from holding military commands, elevating equestrian officers instead. Modern scholar Pat Southern credits these reforms as foundational to the late Roman imperial system built on by Diocletian and Constantine I.
Gallienus was stabbed to death by Cecropius, commander of the Dalmatian troops, during the siege of Mediolanum in 268. Cecropius lured Gallienus out of his tent without his bodyguard by spreading a false report that the besieged forces were breaking out. Ancient sources agree that most of Gallienus's own officials were party to the conspiracy.
Gallienus issued the first official Roman declaration of tolerance toward Christians following the capture of his father Valerian in 259. The decree restored Christian places of worship and cemeteries, effectively recognizing the property rights of the Church. It did not, however, make Christianity an official religion of the empire.