Sounds like he was never conditioned around such small, peeping things and will to be trained how to be around them safely. Always remember that dogs (ALL dogs) no matter how mild mannered they may seem, are PREDATORS by nature, and geese are PREY. There is no avoiding this fact, but you can try and teach him.
I have two dogs. A lab who has had EXTENSIVE training and has been a therapy dog for 8 years. I also have a border collie, who's a "typical" farm dog. Which of these two do you think I allow around Cas? Only my lab, and even then, it's strictly supervised.
LGD's are highly trainable and eager to please. I would suggest you work with him now since your long term goal is to breed geese, yes? That means you will always, at some point in time, have little ones around. And he must learn how to be around them safely, or not at all.
If I were you, I would pick up a pet store product called "The Gentle Lead". It's NOT a muzzle, but is a strap harness that goes around the dogs muzzle (does not affect eating or drinking at all) and a lead attaches to it (under the chin) giving you direct control over your dogs head. When you GENTLY tug the lead, his head is lowered and turned towards you as a matter of correction. It's the only PETA accredited guide harness for dogs.
With the harness on him and lead in hand, put him in a "down, stay" position (lay him down and have him stay) at your side. Have another person bring a gosling near, but not TOO near. If he tries to lunge or snap, correct him. When he sits calmly even if the gosling comes closer, reward him. LOTS of praise.
Source: I used to train dogs.