Can I use the Pass for a return trip Greece- Poland?

Read before: this is a 1 country pass-and thus ONLY valid in that country. And as PL has rather quaint and specific rules, in THAT country it is only available on around half of the trains-but all longer distance IC by PKP that serve all main towns are included-you have to make seat REServations on all of them. That is free when you go to a counter inside PL. PKP also does its own passes for midweek at even much lower prices.

Local all stops trains are mostly done by PolRegio, it also accepts these passes (though most conductors have never seen them). PR also has very, very cheap 3 days in 1 month pass for <20€, valid only on their own services (regioKarnet) or for a whole weekend.

I guess you are young and want to travel. If you make friends with PL students when there, they will tell you you can use half fare. They can, you not, as this is only when you really study in PL and have that card.

You are probably confused with the GLOBAL pass that is only valid in your own country for 1 trip out till border and 1 for coming back. or to an airport to fly out/in and all of EUR.

WIZZair offers very cheap flites in East-Europe and serves PL very good. Seat-only prices from 9.99. PL still has no €, it uses zloty (PLN).

As for now you cannot even go all the way by train as the over the border service from GR to Macedonia and again from MAC on to SErbija and then to HUngary do NOT have trains for now. You have chosen, if you really want to go by train, a most difficult and very slow route.

The many young people from west-EUR who want to visit your country by train almost all go via Italy and then a ferry to Patras. You could do that too if you would like to see more on the way.