I raised four goslings in 2009, three ganders and one goose. Charlie was the oldest and by far my favorite, however, he passed away that December after being hit by a car. Marco, Polo, and Shadow (three siblings) were left. That following January-February after Charlie passed, Marco and Polo reached sexual maturity and became INCREDIBLY aggressive. They lived at the park across from my house and wreaked havoc on humans and birds alike. There were several other older ganders at the park, and Marco and Polo wasted no time in getting around to beating everyone up. Polo was the dominant brother, and Marco backed him on everything. I had several physical altercations with Marco and Polo during this time. The trio could be in the water 20 feet out, and if I walked past, Polo would charge across the water, neck extended, until I turned to face him. On land, the two ganders would charge me from a good 50 yards out. I put Polo in his place several times, grabbing him, laying his neck out flat on the ground, and straddling him on my knees (allowing some of my body weight to rest on his back). Of course during this time, I had to always watch out for Marco because he would become furious when I touched his brother. Polo and I had it out several times, and he continued to pursue me every couple of days. Finally, I decided I was going to initiate the attack. I'm sure I looked like a lunatic to the other park goers, but I came across the small flock from a distance, and I charged them. Polo and Marco charged back, as they usually do, but this time, Polo didn't stop. We slammed into one another and I got him down on the ground in a subordinate position. The whole while, I had to hold Marco off with one hand because he was in the process of tearing into me. This is where things get interesting... Another gander, whom I named Stripeneck at about age 10 because of a few horizontal stripes on his neck, who had been put out by the dynamic duo took the opportunity to surprise attack Marco! Marco was completely taken off guard, and without his dominant brother, he panicked. Stripeneck sent Marco packing, but the minute I realized Polo and sent him on his way, he called on a full fledge attack on poor old Stripeneck. I felt terrible for the old gander, but Marco and Polo proved too much for him. Polo and I had no more trouble after this fight, but I eventually had to move him to a different pond because Parks and Rec had gotten too many complaints about their aggression. Marco calmed down considerably after I moved Polo, so everything worked out.
I agree with Olive Hill, geese are incredibly physical creatures. Dominance, hierarchies, and doing as you're told are incredibly important to them. I may look like a nut, but I treat them exactly how they treat one another. If they are out of line, they get hissed at by me or I give them a mock charge into the water. Even though they live at a public park, they respond to my voice and image, but they know the rules. You can't train them in the same sense you can train a dog, but being dominant is a universal language, especially to animals, and in particular geese. If you want them to be happy, healthy, and respectful: PUT EM IN THEIR PLACE. They won't hate you for it, they'll be wary of you, but still loving creatures. You don't have to hit them. Olive gave an excellent description of how to handle it, and it's exactly how goose fights play out.
