I normally take the train (a good option for Gdynia also). When you exit through the arrivals door walk all the way to the left end of the concourse and you'll see some escalators going up, and from there you can get to the platform for trains to Gdansk. There's an automated ticket machine just by the bottom of the escalators but you can also just buy tickets on the train (Poland is much more reasonable than the UK for things like this).