I saw a post somewhere explaining this, apparently the alignment of the gate has an effect. Apparently the gate must face outwards (towards the purge) for the purge to work. If the gate faces inwards, the purge would fail? I havent actually tested this but I intend to.
Problem is that many gates are very symmetrical, and its difficult to see which way is forward. If you set thralls to guard the gate, they can only be placed on the outside of the gate, so this will tell you which way is “forward” with the gate.