Gallia Imperium is an alternate timeline that splits from reality beginning in roughly 261 A.D. The Romans are not able to reconquer Gaul; the Gallic Empire is the byproduct.
The Roman Crisis of the Third Century continued as Emperor Valerian was defeated and captured by the Sassanid Empire of Persia, leaving his son Gallienus in very shaky control. Shortly thereafter, the Palmyrene Empire, which came to encompass Egypt, Syria, Judea, and Arabia Petraea also broke away.
This timeline deals with the possible reality of a successful Gallic Empire which might have resulted from the Crisis of the 3rd century.