I'd personally consider Flamethrower over Shadow Sneak, as Gourgeist doesn't quite have the power to revenge kill, and Flamethrower lets you instantly get rid of Forretress even with Gourgeist's paltry SpA. Will-O-Wisp is good enough against Forry in most cases though. You could also look at a Resttalk set if you want for some recovery. 252HP/252Def may work better against physical ground types if you so desire.