basically collect the golden gnomes which are hidden all around the surrounding area i collected 9 at the time and noticed a payphone ringing answer it and dialogue pops up skip it as its aloud of blabber, soon after another phone call with the same payphone will happen answer it and yet another blabber convo will happen after the dialogues finished go over to the docks and there will be a gnome bomb pick it up and make your way down to the sewers. where you will see a ghost gnome where the gnome bomb your holding needs placing, place it and the metal bars will explode and reveal a secret passage, follow the pathway round and you will see a giant golden gnome the trophy will pop.