Thanks everyone for your thoughtful replies.
Can someone point me to a reference in the rules that talks about when Targeting is needed, or that only Ranged attacks need LOS? I agree that it makes sense (which is why I also mentioned that the attack is neither a melee nor a ranged attack), but the Targeting topic makes me think that all attacks must have LOS. From p. 15:
"Targeting
In order to successfully target an object, 3 conditions must be met.
1) You must be able to “see” the object – trace LoS (Line of Sight) to the object.
..."
I guess the way to look at this is to say that the specific wording on the card overrides the more generalized rules in the rule book. Thus, there is no possibility of being able to hide behind walls, etc. to avoid the attack. If the creature is in range, it is hit by the attack.
Oh, and to get this attack off, it requires two Mage actions... one to cast the original spell, and a second one to open the gate... plus a total of 24 mana... 12 + 12. Therefore, it should be a powerful attack (though you also get a fairly sturdy conjuration that gives Demons +1 dice and an alternate entry location for them).
Thanks again!