Please do not buy geese as watch dogs. Sure they will alert you to predators but they will also alert you to sticks, songbirds flyin, planes, cars, bikes, etc. You'll be walking out every five minutes.
The loudest and most intimidating are usually Chinese and African, but they are fog horns.
Not only will they alert you to EVERYTHING that is suspicious (which is anything that moves because they are a prey animal) if there is an actual predator and you don't get there in time you'll have dead geese. They are BIRDS. They stand no chance against dogs, cats, foxes, raccoons, coyotes, etc.
And if you're looking for watch geese, I'm assuming you'll keep them out at night so they can keep an eye out? That's unacceptable. All poultry should be locked in a secure shelter that no predators can get into because, again, they are a bird.
So if you are only getting geese only to look after your other poultry, just don't get them. There are many other tools to ward off predators. Hot wire, predator lights, different scents that deter a specific animal, etc etc.