I have an Aussie Shepherd...while I love him, and he is energetic and playful and sweet, he is no fierce flock protector! He is either A) indifferent to the flock, B) throws his tennis ball at them and barks, tips his ears up, and fully expects them to throw the ball, worst, C) he suddenly sees red and chases them!
I have now lost 3 chickens...1 was a RI Red who vanished mysteriously, like your Gypsy- no fuss, no feathers, just never came home to roost (it's been 2 months now) 2 bantams, both to hawks.
After not being able to convince my hubby that we needed a Great Pyrenees, I decided to get a rooster. So far, that has been a good solution. He keeps the flock together, there is no more "hen pecking" going on, and the newer girls have been accepted.
If you are seriously thinking about a dog, do a little research in the Great Pyrenees- they are breed to be livestock guardians. There are some rescue sites available as well!
And I'm very sorry about Gypsy. We love our girls, too!