There are regular trains to Gernika from Artxuri Station (at least once an hour, normally two an hour) on Euskotren. It is the line that runs to Bermeo. The journey time is about 45 minutes.
Hint. If you are travelling by metro in Bilbao, get off at Bolueta, as this is an interchange between Euskotren and MetroBilbao.
GUERNIKA is located in a perfect place where the ria springs and easily you can get Bilbao or the beaches northwards with their beautiful towns