Attacks » Gathering Footsteps
What happens to Piloswine's "Gathering Footsteps" attack if Mamoswine has a Float Stone attached to it?
Float Stone reduces Mamoswine's Retreat Cost to zero, so you would not get to add any damage from it for Gathering Footsteps.
Source: BREAKthrough FAQ (2015-11-19), TPCi Rules Team (2015-11-19)