Two options
If you load any campaign map that has a door that opens you will find there is a "Send actor message" trigger that will work for you, this can be a bit complicated and makes you need to know the ANIMATION BIRTH/STAND/DEATH EVENTS the actor uses. And you could use the campaign maps as a reference.
THe other option I find more simple however.
There is a METAL DOOR STRAIGHT unit, supplied with the Campaign Dependencies, give this unit to a neutral player and set it to invuln using either a trigger or a behavior, and then use triggers to "Order unit at location - Open Gate" or "Close Gate" (these are an ability the unit has already so you dont have to do any extra work.)
Simple as that.
This gate also prevents Line of Sight and pathing while its closed. so its just as good as a doodad, only better, because you could also use triggers if its not invuln, to open after a certain amount of damage.