According to Sikhism the Tenth Gate can only be opened by the Guru's
grace and is vital for Self-realization and enlightenment.
The energy channels of the Ida, Pingala and Shushmanaa:
These three dwell in one place.
This is the true place of confluence of the three sacred rivers:
This is where my mind takes its cleansing bath.
O Saints, the Immaculate Lord dwells there;
How rare are those who go to the Guru, and understand this.
The all-pervading immaculate Lord is there.
What is the insignia of the Divine Lord's dwelling?
The unstruck sound current of the Shabad vibrates there.
There is no moon or sun, no air or water there.
The Gurmukh becomes aware, and knows the Teachings.
Spiritual wisdom wells up, and evil-mindedness departs;
The nucleus of the mind sky is drenched with Ambrosial Nectar.
One who knows the secret of this device, meets the Supreme Divine
The Tenth Gate is the home of the inaccessible, infinite Supreme
Above the store is a niche, and within this niche is the commodity.
Raam Kalee, The Word of Baynee Jee, Adi Granth, p. 974.
Please read deailed explaintion about Dasam Duar in Bhai Sahib Randheer Singh's book "Anhad Shabad Dasam Duar"
Some Excerpts from this book:
There are two types of liberation. The first one liberates from rebirth cycle of transmigration and attained by envisioning The Effulgence of soul being. The second liberation is riddance of the invisible and causative bodies, on envisioning The Full Glory of The Supreme Soul.
Soul being is revealed when mind becomes motionless and breath is stopped, while Supreme Soul revelation is on the opening of the Dasam Duar.
To clarify further, it is brought out that Guru Nanak was not merely a son to Mehta Kalyan Das and an ordinary person. No common person becomes immortal whereas He is described thus in Revealed Gurbani:
The invisible and causative bodies are rid of on envisioning The Supreme Being when The Dasam Duar is opened. The strong Door opens with the key of Gur Shabad, 'Waheguru', Naam. Naam is available with Satguru, Guru Nanak, The Manifested Form of The Supreme Being.
You are The Satguru for all The Four Ages, Yourself as 'Parmesar', The Supreme Being. 
The above quote clearly emphasizes that Guru Nanak is Satguru for all times to come from the very beginning and He is The Manifested Spirit of The Supreme Being, playing the role of Satguru at all different worlds. It is a serious lapse to think of The Supreme Redeemer, Satguru Nanak Dev Ji, as limited to a small Nation of Sikhs instead of belonging to the entire mankind now and forever!
Coming back to Naam Key, bestowed by Satguru, Guru Nanak Dev Ji. Even though He is not in our midst as a body, He enshrined His Spirit in Gurbani of Sri Guru Granth Sahib before leaving. Now His Spirit in the Presence of , through the medium of Five Beloved Ones, bestows Gur-Shabad, Naam, on the spiritual seekers. This process shall continue forever blessing all the deserving persons of this world.
In confirmation of the truth that before coming to this world, Satguru Nanak Dev Ji was engaged in the work of redeeming persons on other worlds, we have the witness of the son of Pir Gulam Dastgir. He was taken by Guru Nanak Dev Ji for a visit to countless Galaxies and underworlds. Our scientists have still to discover a planet that is inhabited by human population.
This leads us to conclude that all persons desirous of their redemption should present themselves before The Five Beloved Ones in thePresence of Sri Guru Granth Sahib, to get bestowed with Gur- Shabad, Naam. Thereafter, ceaseless devotion to the practice of Naam is efficacious in the opening of Dasam Duar.