Geese can make excellent "watchdogs" or guards. In fact, geese are used to guard businesses such as whiskey warehouses in Scotland and military facilities in Europe. Geese are loud and quick to respond when they hear the slightest questionable noise. Geese naturally understand, without any training, that any people and animals living on the property are part of their flock. They tend to have no fear of challenging a human or animal that intrudes on "their" property or that threatens "their" people and animals. A flock of geese may be an unusual method of home security, but they can be effective without any special training.
Get the right breed of goose to protect your home. Chinese geese have the temperament of loyal watchdogs. African geese can also make good guard geese. Both are confident, large and imposing to strangers. You can get geese as gooslings or adults; however, don't expect them to offer much protection until they are full-grown.
Get at least two or more geese. To protect a property, the more geese there are making noise and running at an intruder, the more effective they will be.
Introduce friends and family that visit regularly to your geese so that they will be accepted and not targeted by your flock. You can do this by having people hand-feed your geese and spending time with them.
Provide the best of care for your geese. This includes healthy food and lots of outdoor space to roam. A healthy goose is more likely to be confident and protective. They can eat grasses and weeds found in the yard as well as a formulated waterfowl pelleted food.
Use caution, and fences if needed, to protect people from your geese. Geese tend to be indiscriminate in their aggression toward strangers or unapproved guests. They can and will bite, causing serious bruising.
The book "Domestic Geese" by Chris Ashton is an excellent reference that covers everything from food to housing to medical care.