Looks like Neguri is the closest metro stop to the port (about 25min walk accdg to google maps) and to get to Guggenheim the stop is Moyua then 9 minute walk to Guggenheim. Looks like metro rates are very reasonable, around 1 Euro for 1 way trip (although Google says 1.9Euro). Hope this helps.
We are stopping there in October and this is making me rethink that the metro is an option vs other taxis/transfers as we are mobile and can walk. Not sure about if touring the town ourselves makes sense