Once a player has acquired the Khazra's Staff, and then heads to the gate to open it, the gate opens, and while players, goatmen and other monsters can pass through it, the "blocking volume" aspect of the gate does NOT vanish. This prohibits certain projectiles from being able to pass through it - projectiles treat the gate's space as some sort of impact zone to connect with, like a wall. I can personally confirm this for:
The Wizard's "Arcane Orb." A Demon Hunter's basic projectile attacks (from dual one-handed crossbows).If it happens with other projectiles or mechanics (ie, a Barbarian's Leap), I do not know.