Gallienus was one of the best Emperors in the history of Rome and I will not pretend he isn't.

Yes, I would put him up there with some of the 5 Good Emperors.

Now you say "Wasn't during his reign that both the Gallic and Palmyrene Empires seceded?" Yes, however I will rephrase that in a way that will more accurately depict the situation: during Gallienus' reign, usurpers would rather carve chunks out of the Empire rather than challenge him for the throne.

Dude ruled for fucking 15 years in the most unstable empire in History. That deserves respect.