By reverse you mean any Greek philosopher visiting India not Plato specifically?

In the fifth century BCE Scylax of Cariander was, according to Herodot, a Greek or at least western captain who was send by the Persian King of Kings Darius to explore the Indus river. He then returned by sea to Egypt. Herodotus writes:

Of the greater part of Asia Darius was the discoverer. Wishing to know where the Indus (which is the only river save one that produces crocodiles) emptied itself into the sea, he sent a number of men, on whose truthfulness he could rely, and among them Scylax of Caryanda, to sail down the river. They started from the city of Caspatyrus, in the region called Pactyica, and sailed down the stream in an easterly direction to the sea. Here they turned westward, and, after a voyage of thirty months, reached the place from which the Egyptian king, of whom I spoke above, sent the Phoenicians to sail round Libya. After this voyage was completed, Darius conquered the Indians, and made use of the sea in those parts. Thus all Asia, except the eastern portion, has been found to be similarly circumstanced with Libya.

From this, we can at least conclude that India was known during the lifetime of Socrates. And furthermore, the Achaemenid Persian empire stretched from the Mediterranean to the Indus at that time, providing relatively safe conditions for travel. So it is plausible that at least one Indian somehow ended up in Athens. Though it should be mentioned that this was as likely the result of slavery as it may be the result of curiosity.

Source: Raimund Schulz, Abenteurer der Ferne, Klett-Cotta, Stuttgart 2016.

Can you point me towards where that is stated? The gymnosophs were Indian philosophers (it is plausible they were Jains) who made a splash in the Greek intellectual world, but that was after Alexander. I'm not aware of any before.