In you particular case, the door is fine. The "bottom" brush making up the wall is half hidden by the func_Detail stairs. It'll be beneficial to cut this wall vertically at the end of the stairs, and texture the hidden half NoDraw, as it'll be hidden by the func_detail at all times.
HINT splits visleafs. If you do not understand it, avoid it (for now at least).
I have no knowledge of Garrys mod, or merging content from other games. I tend to stick to which ever game im making it for and use what's available. I hope someone who has some knowledge in this area can help further.
I have a little problem, I place some doors, I think they are already in the previous file I uploaded
I didn't check the doors actually I always nocliped into the building, when I tried to use the door they were working really weird. When they appeared closed they were actually open and I could walk through them, and when I pressed E the door appeared open but i coudln't go through. How to fix it?