Did you mean - " you are going to Gdansk, how easy travel from Gdansk to Lithuania"?
If I'm right, you can try this http://www.itisnet.com/english/e-ce/e-poland/e-gdnsk/e-t-gdn.htm is a train and ferry time table. Hope this will help.
Gdansk is in my traveling list and I'll going to Kaunas and Vilnius, Lithuania soon.
Have fun there.