I believe cash games are illegal in Poland. There are some tournaments in Warsaw and Wroclaw, but I have no idea about Gdansk. I went there years ago when I wasn't playing much poker.
In Eastern Europe casinos are often found in Hotels, such as Hilton, Novotel, Sofitel etc. So it might be worth googling or even calling a couple of hotels to ask. Had a quick scout online and I can't find any up to date info I'm afraid
Sopot is a seaside town just a few miles away to also try. There is a Grand Hotel there but I don't think there is a casino in there.
You should go to the shipyards in Gdansk if you have any time in between drinking, really fascinating history!
Have a good time!
Another vote for Sopot.
A few years ago since I was there, and fairly sure I went to a Casino in the Grand Hotel.