Short answers, pulled from the story.
Marcus Cassianius Latinius Postumus emerged from the Batavian State, a region in modern-day Netherlands. Scholars hypothesize he was born in Deuso, a town near Batavian territory.
The revolt began when the army in Gaul rejected Gallienus's authority around mid-260. Soldiers proclaimed Postumus emperor after he besieged Colonia Claudia Ara Agrippinensium and ordered the deaths of Saloninus and Silvanus.
Postumus established his capital in northern Gaul with cities like Colonia Claudia Ara Agrippinensium or Augusta Treverorum likely serving as seats of power. He assumed office of consul alongside Honoratianus while administration rapidly became Gallicized.
Postumus ruled for four years starting from 260 until he was killed by mutinous troops within months of assuming his fifth consulship on the 1st of January 269. His rule ended after soldiers turned on him following the capture of Mogontiacum.
Postumus made clear priority was Gaul, not Rome because marching against Gallienus would expose heartland to Germanic tribes. He hoped for official recognition from Gallienus while keeping his power base firmly within western provinces throughout conflict.