Latest news shows that the World Bank now ranks Poland as the 18th largest economy in the world.
You are in....in the EU slot!
en.wikipedia.org/wiki/G-20_major_economies
g20.org/about_faq.aspx
I don't think that Poland's economy isn't good enough it seems rather that the economy isn't everything for the G-20.
I rather think that it will be hard for yet another EU-member state to get yet another "extra-place"....as the G-20 are already 19+EU.