I don't know of any source that discusses Druids coming to Greece to teach and learn, however, there certainly were plenty of opportunities for Greek and Celtic teachers to interact. For example, the Greek settlement of Marseille in southern Gaul was founded around 600BC and the famous Roquepertuse temple site just north of the town is thought to show Greek influence in its stone-masonry.
Through this link, many writers have speculated on the possibility of Greek influence on Gaulish religious beliefs (Which I imagine is probably what you've read), especially Pythagorean ideas considering Diodorus Siculus wrote in his Histories, V, 28, 6
The Pythagorean doctrine prevails among them (the Gauls), teaching
that the souls of men are immortal and live again for a fixed number
of years inhabited in another body.
Other than that there was significant Greek trade in the Western Mediterranean (The Cambridge Ancient History), the 4th and 3rd century BC penetration of the Balkan Peninsula (Cunliffe, Barry (1997). The Ancient Celts. Oxford: Oxford University Press pp. 80–81), and the colonisation of Galatia by the Tectosages, the Trocmii, and the Tolistobogii immediately after(Strobel, Karl (2013). "Central Anatolia". The Oxford Encyclopedia of the Bible and Archaeology.) All plenty of opportunity for Greek and Celtic philosophers to mingle.