During the latter half of the 3rd century the Roman provinces of Gaul, Britain and Spain broke away under Postumus. Under Postumus and his successors the Gallic Empire didn't last for long but what if it had??
They'd have to develop a whole new state ideology first. THe only reason the Gallic Empire stayed Gallic was becxause they couldn't take Rome and Rome didn't have the resources to take them. For this state of affairs to continue, there would need to be an ideology that allowes a part of the Empire to remain apart for a longer period of time. Perhaps a concept of co-emperorship, or a Hellenistic-model kingship.
Territorial co-emperorship of two or more not mutually allied emperors at this point in time could have interesting effects down the line.
Now if you want the Gallic Empire to survice you may try :
- A strong emperor taking over after Postumus.
The problem with Laelianus is that he is pretty much an unknown quantity. We don't know anything about him except that he revolted against Postumus and failed.
IMO the best option is Postumus living longer.