Another point/question. I'm pretty sure that you have to interrupt GoS if Omri is used. You can't use special initiative against Omri himself because he is not the one doing the discarding. Am I correct?
Yes.
Essentially, if Omri activates Gates,
that is the card causing removal, and you must target
that card. If your card does not interrupt or negate
that ability, it cannot be used, so something that only hits Omri wouldn't work.
EDIT: Just wanted to clarify that if you negated Omri
outside of special initiative (let's say more than one hero was in battle, and you played a negate with the one remaining), then everything would cascade and indirectly negate Gates. If Omri is negated, no Samaria is put into play, Gates is not triggered, and the decrease does not occur. It is only in special initiative that Gates must be targeted to stop the effect, you can still cascade it outside of special initiative.