Ok thanks-So I will try to negitiate for 5800-6000 gross.
It doesn't work like that. Poland is not a rich country, so you cannot simply get whatever salary.
If you get 800eur net in Athens, then it's 3341PLN and in Gdańsk you will get 3201PLN. You will move to the city with much worse weather and less beautiful than Athens.
Decide yourself weather it has sense by looking at this page:
It's Gdańsk here, because of no data for Gdynia, but it's the same agglomeration, so only apartment prices should be little lower in Gdynia.
About salary. Average for Poland is around 4000PLN gross, for big cities perhaps 5000PLN. Most of people earn less than average.
Greece was much more expensive than Poland few years ago, but now because of the crisis renting and restaurant prices are comparable to Poland. You will pay in Poland less for grocery (in Greece they cost more than in Germany), going out to the pub, restaurant. Public transport is cheaper.
If you're not an programmer then probably the job is some call center. You should decide yourself if it's any better from what you do there in Greece. Any surplus which you theoretically save in Gdynia you will spend for tickets to Greece.