what door model exactly? I don't think I've seen any doors in watchtower that couldn't be made of brushes. Are you sure the door you're thinking of is not?
If you are talking about the setup gates that open once the round starts, make a prop static and under the props_gameplay folder type gate. I guarantee you will find it. Even if you just simply typed gate into the search box, you should find it
The only doors that open once are the setup gates at the very beginning of each round. There should only be three with a set of top and bottom for each. If that isn't the door you are looking for, then the door you are looking for does not exist in that map