Barad Gúlaran is the ancient tower of the Witch-king. The entrance is located at Minas Deloth (6.6N, 21.8W) in Nan Gurth located within north-east Angmar.
To get there from Gabilshathur, head north until reaching the road that branches off into Imlad Balchorth or Nan Gurth. From there, take the road to Nan Gurth, but head to the right halfway along the road. Proceed through some mobs while staying near the cliffside on the right. This will bring you to a path snaking along the cliffs. Keep following it until you are just south of Minas Deloth, and it will branch off into a path that leads straight up to the door, which is the instance entrance.