I third locking it. Yea it's a pain in the ass, but it gets the message across without having to count on someone reading a sign, and then actually abiding by it. And as Jazzy said, maybe you could ring up some type of buzzer that goes down to the work shop or the house, or an intercom system. Also, sorry if I missed this, but if the only problem is the gate getting closed behind, could you not put a spring or something in the gate so it swings shut by itself if that's the only concern?