The "curb" command is wonderful for preventing your dog from bolting out a gate or door, but it doesn't eliminate the importance of common sense. All too often we rely on a command alone, or we take for granted that our dogs will do the right thing (in this case, stay back when you open a gate or door). Suddenly the illusion is shattered as the dog bolts away from us and down the street after a cat who caught his eye.
Even more important than using a command or a signal to "curb" at a gate is this: keep your eye on your dog as you open the gate.
Let's say you're in the back yard with your dog. You're about to open a gate to the front yard so you can exit, but you want your dog to stay behind. As you actually open the gate, position yourself so you are blocking the opening, and turn so you can see your dog the whole time you're moving through the opening. Many a dog has squeezed past an inattentive owner and darted away to freedom. This need not happen if you simply keep your eye on your dog.