Another member here on spn findingmyway explaind the meaning of tenth gate very well i think
"The dasam dwar is not mystical but refers to the brain. The brain is the site of consciousness, integrating physical and emotional information and controlling how we behave both physically, morally and spiritually. Therefore the brain is the door to our consciousness. The Guru's always tried to make everything accessible to all regardless of their background so it doesn't make sense that such an important door should be so mysterious. Without a brain, spirituality cannot be accessed so we all have a dasam dwar. However, the choice is whether to open it to spirituality and a Sikh way of life."
Sir, this is quite interesting and thank you for sharing it. The dasam dwar cannot be a physical gate as it would be no different from the other 9 gates of the body. Well certainly not enough to afford it an seperate mention in Gurbani, and an exalted status in Gurmat.
"Mysterious" may be a little inappropriate to describe the dasam dwar, as Guru Granth Sahib does not cloud it under a veil of secrecy or mystery. Instead Guru Sahib desctribes it a goal, or stage to be worked for. Guru Sahib is very explicit when talking about Dasam Dwar in Anand Sahib.