Gan Men is located on the Small Intestine channel, 6 cun proximal to the wrist crease. There's a trick to locating this point.
You want to position the patient with the palm down and needle the point on the Small Intestine channel below the ulnar bone.
This point is often needled with Chang Men, which is 3 cun proximal to the crease of the wrist on the Small Intestine channel (or 3 cun distal from Gan Men).