Gatekeeper is meant to prevent a wide-ranging attack. If you've got someone custom crafting an attack vector for you specifically, you've got some serious problems. I don't think Gatekeeper adds any more false security than an antivirus app would, and I don't know of any software that'd prevent an attack like this.

At best, it adds a small amount of information for authorities to try to track the attack. I doubt it'll be fruitful, but it's better than nothing.