Okay I've searched these boards and BGG but though the question has been asked I see no good answer. Specifically what the OP asked in this BGG thread, first post.
http://boardgamegeek.com/thread/413355/zhars-cascading-gates-how-does-this-work
Exactly when do you roll for this effect--right after placing the first gate (and I presume monster) or after resolving the entire first mythos card?