Can I react with Gust before Assault or Mask of Foresight apply at the start of a showdown?

Once a combat showdown starts, attackers and defenders are designated before you get priority to play cards or abilities. That means Assault will already be granting Might before you can play Gust during the showdown. Mask of Foresight is a trigger that goes on the initial chain if its extra condition is met — you can Gust before it resolves, so the target won't get Mask's Might bonus before Gust resolves. One wrinkle: sometimes a showdown can't start yet because the action that created it also created a trigger, such as moving with Cleave. In that case your opponent can react to the move trigger with Gust before Assault applies, because combat hasn't actually started.