I built a fence around Sanctuary and so I think I'm save from getting attacked from behind. I built a choke point at the bridge by deploying machine gun turrets there At this point I expect Raiders to attack. North of Sanctuary I built two doors in my fence as I think it could be helpful to leave Sanctuary to go northward.

Now my quesion: is it possible for raiders to open doors and gates? Or is it only possible for me to open my doors and gates?

Thank you very much!

Yes, raiders can open doors, but put some traps in their way so they will work as both a distraction, and a warning to the defenders, but you could also make this a death trap by setting machine gun turrets that will help the settlers fight back. Don't forget to properly arm the settlers with weapons that'll put'em down!