Make two dummy abilities based on Channel. Make sure they have unique BaseOrderID's and set all special effects to null (unless you want them), set options to visible, follow through time to 0.01, casting type to instant, mana cost to 0, and cooldown to 0.01. One will be "open gate" and the other will be "close gate".
Now make a trigger that detects Event - Unit Starts the Effects of an Ability. In the conditions check to see if it is the "open gate" ability. In the actions do Destructable - Open [your gate].
Now make another trigger that detects the casting of the "close gate" ability and have that trigger close your gate.