Card abilities cannot cause characters to enter play.
Marshaling Action: Choose an opponent whose reserve value is lower than yours. Until the end of the phase, you may marshal characters from that player's discard pile as if they were in your hand.
Does Barring the Gates block Night Gathers?
I interpret 'cause' as 'directly cause'. As in, the resolution of a card ability would result in a character entering play. Night Gathers doesn't put characters into play, so it doesn't directly cause characters to enter play. If you interpret 'cause' wider, as in 'without this ability you couldn't put this character into play', then you'd need to account for discounts and gold-generating abilities as well, no?