If all of you opposed to the gate get together, hire a lawyer, and sue, it'll cost her a lot of money to defend, and I presume that nobody who spoke in her favor are so invested enough in the fight as to share her attorney expenses. I also assume that the city isn't going to take a hand in defending. You might get it removed just because it'll cost her a fortune to keep fighting you.
She can't be sued. She didn't close the alley, the city council did. Citizens can't be sued for making requests.