How early should we arrive?.before it opens early or just within the first hour of being open early?

Show up right at nine. No one else will be there and that is the best experience.

They told me the gate is automated so that it opens/closes at 9am/9pm

Honestly, everytime I go I aim for the time they open only bc when i go, i camp. So i go early to find a good spot. They open at 9am and close at 9pm.