I'd suggest going to Underground Rock Pub in Gdansk, I've been there once and they were playing The Doors all night, there's a lack of such pubs in Silesia ;)
here's the address: Adres ul. Podmurna 2 80-835 Gdańsk it's in the old city section ; o
On this site you'll see a google map with it's location: aktivist.pl/miejsce/localId,4012,underground-rock-pub-miejsce.html
Ucho in Gdynia has a lot of punk, metal and rock shows, check their lineup at ucho.com.pl. Walking distance from the main Gdynia rail station, there's a map on the site. Not strictly Gdańsk, but the 3-cities are well-connected with one another,
Merged: What's a good hard rock / heavy metal themed bar in Warsaw?
I was in Jazz/Rock Club in Krakow last Sat night and loved it. Great music (tool, Rage Against the Machine, Nirvana) and a cool, relaxed atmosphere. I haven't found any good bars like this in Warsaw. Rock Cafe is stuffy, overpriced and full of foreigners.