Lets say it is winter and you have the wall down "If it is Winter, your House Baratheon characters gain the Night's Watch trait and do not kneel to defend".
Can you use the gift "Marshalling: Kneel The Gift to reduce the cost of the next Night's Watch character you play this phase by 1" to reduce the cost of your baratheon characters?
Probably the answer is no since you gain the trait upon entering the board but still its worth asking