Right, so especially because the gate was greyish and the marks on your car are greyish.....its most likely paint from the gate that got stuck to your car. Buy a polish like this and use a microfiber towel to rub the grey paint off of your car.
https://smile.amazon.com/dp/B004HCOE8Q/ref=twister_B088XQNSW5?_encoding=UTF8&psc=1
Try car polish. Something like this: https://www.amazon.com/dp/B004HCOE8Q