The mean average salary in Poland is blown out of proportion by extremely high salaries of people in management and government jobs. Meanwhile, finding a 'common' job that pays anywhere close to the country's average is very difficult.
I believe using median average would show the more realistic salary level, and what common people can actually afford - but my search for any sources that would give median salary as opposite to mean average have proven unsuccessful so far.
Both gross income and net pay are okay - converting between one and the other for incomes in the lower range is rather straightforward. A source of this data, that either records it over time or gives current value, updated over time would be preferred, but in absence of such, a momentary, current/recent data point would suffice.