Ughhh, I'm here. Yes, no, Grass Knot is never a move you'd want to use on Gengar. Gengar has respectable Special bulk and Wiggly isn't overwhelmingly powerful, so it's not great to begin with, but 60 base damage on a resisted hit (due to Poison typing) is nothing. Gengar has an overwhelming advantage here, resisting Fairy moves as well and being able to deal great damage, possibly OHKO, with Sludge Bomb. Re:Gengar being heavy - its pre-evolutions are tied for the lowest weight of all Pokémon, 0.1kg or 0.2lbs; Pokédex statistics don't go lower than that, so we don't know precisely. Gengar's presumably gained more corporeal presence in evolution, particularly evidenced when it Mega Evolves and loses its power of levitation for the first time.