Yes they are under the destructables section, but you have to each "enable" or "disbale" them. I think when they are enabled, they are close, and disabled are open, i could be wrong on that one though.
As for your vehicle question, you may have to do a trigger which replaces the unit with another unit. Like you can say
Unit Enter vehicle1 replace vehicle1 with vehicle2
just make vehicle2 be able to move and and have all your attack damage you want on that and make vehicle1 not movable w/ no attacks
Then for exitiing you can say Unit unloads vechile2 replace vehicle2 with vehicle1
this is just the basic conept of the trigger, there are other things that would have to be included. but that should get you started
Try to open the warchaser & look the gates category in the trigger editor. Suppose you dont want any requirements for a gate to open then just remove the condition & some of the actions that concerns the item. for the closing of the gate you can just change the event into a unit leaves a region & the action will now be close gate.
Edit: Also for the vehicle thing, you can try to modify the ability mount hippogryph & w/ that you will not need any trigger.