He beat back multiple invasions, He had to deal with usurpers left and right, and set up the empire so Clodius Gothicus and Aurelian only needed to walk across the goal line. (Yes, I'm simplifying reuniting the empire.) I mean Aurelian had the time to go to Palmyria and wage war. How could he have done that if Gallienus hadn't started (not finished) some sort of stability on the Danube. Anyone agree or disagree?
If I could wrap his reign up in one word, it would be "beleaguered." His father had been captured in battle, Frankish invasions from both land and sea were a problem, the so-called Gallic and Palmyrene empires had split off from Roman authority, and he may hold the record for highest number of usurpers during his tenure.
He did reverse the rabidly anti-Christian policies of his father and Decius.
Gibbon criticizes him as being indolent and useless, and that opinion has stuck for generations of readers. I've never bought that, though. Yes, Aurelian got to be the hero for putting down Tetricus and Zenobia, but Gallienus had plenty of his own problems to deal with.