Though you have mentioned that everyone has done illegal construction in your building, you have not specifically mentioned whether the gate that you had erected is also illegal or not.
You can approach the appropriate court for challenging the notice given to you by the concerned authorities for demolition of the gate and for getting stay against such proposed demolition. You may also point out that there are several other illegal constructions in your building and that the gate existed for 30 years, and also that action against illegal constructions should be taken without discrimination.
For detailed grounds to be taken, you may contact some local lawyer and show him all relevant records / documents / details since that requires a detailed study of the relevant facts.
Dr. Ashok Dhamija is a New Delhi based Supreme Court Advocate and author of law books. Read more about him by clicking here. List of his Forum Replies. List of his other articles. List of his Quora Answers. List of his YouTube Videos.