In Hellenistic warfare, I would say VERY common. Mind you, it's a little out of my usual area of interest! Apparently Galatia was a heavy recruiting ground for Alexander's Successors, to the point where a whole new type of infantry became common, the thureophoros or "door carrier". From the Greek word for that tall oval shield, of course. They were simple cheap infantry, spear-and-shield men, generally not heavily armored from what I've seen but helmets were "standard equipment".
Though by some point I suspect that a lot thureophoroi were simply whatever locals were available, not necessarily Gallic or Celtic at all. We know there were forces of Greek thureophoroi, for example.
Before that, sure, Hannibal is only the best-known example of a Mediterranean general hiring Celts. The Romans certainly used them as allies even before enlisting them as auxiliaries. It wouldn't surprise me if a lot of indigenous Celtic warfare included bands of mercenaries, fighting for whichever tribe or warlord would hire them.