Geese will deter predators by being the first to get eaten, but that's not an ideal situation. I had "guard" geese, but they were useless. I had more problems from the geese than I did any other predators. The geese routinely attacked smaller birds around the yard, caused numerous fights, were ear-splittingly loud (except when a duck was actually getting killed by a hawk- then they were silent), and ferociously territorial. When they started biting me and chasing visitors they had to go. I still have one goose from that lot and she's pretty good, but I had to cull 3 others to get one that was an appropriate guardian.
I wouldn't recommend using geese as flock guardians, though.
I have Tufted Buff (calmer geese) and they free range in the fenced yard with my runner ducks, I have never had an ariel attack. (I live next to a National forest and hay fields).... I also read that a survey shows people are more afraid of geese than dogs...
I had geese years ago and a gander will sometimes go after someone during breeding season,if they get too close to the nest or wife. Some breeds of geese and or individuals will/can get aggressive . If handled wrong, you can make them mean. They mostly bluff....They will make a racket if something or someone approaches they don't recognize...hence they will alarm for you. We found that geese let you know about strangers quicker than a dog sometimes.....But no they aren't guard dogs ,like LGD, but they don't cost 1000$ either and are interesting in their own right. Some owners of chicken, duck flocks say that after they added geese or turkeys to a flock , they stopped losing birds to raptors.... and may deter smaller predators, like cats...(neighbors cats leave our property quick when I let the Buffs out.and our cat won't go hunting in the back until I put the geese up.
However I wouldn't advise letting guest just go into a pasture during breeding season without being forwarned and accompanied by me. Small children can be at risk for some gandars to chase (mainly because they run).