Short answers, pulled from the story.
Postumus founded the Gallic Empire in 260 after he besieged Colonia Agrippina and executed Saloninus and Silvanus. He served as governor of Germania Superior and Inferior before declaring his revolt against Emperor Gallienus.
The Gallic Empire functioned de facto as a separate state from 260 to 274. It ended when Aurelian defeated Tetricus at the Battle of Châlons in 274.
At its height the Gallic Empire included territories of Germania, Gaul, Britannia, and Hispania. These regions formed a breakaway region that operated independently without attempting to conquer Rome itself.
Numismatic evidence shows Postumus held the office of consul five times. His regime established parallel institutions modelled on the Roman Empire's central government including an own Praetorian Guard.
Gaius Pius Esuvius Tetricus was the last emperor of the Gallic Empire before he surrendered to Aurelian. He ruled until 274 when Aurelian defeated him at the Battle of Châlons.