If your gelding bullies other horses when he's turned out in a field, follow this advice from equine behaviorist, Garry Bosworth.
Q: My gelding's turned out with two other horses, also geldings, and he bullies them. Why does he do this and is there anything I can do to stop him?
A: Aggression is a natural and necessary trait for horses. It's enabled them over many generations to protect themselves from predators and to defend their offspring (mares) or harems (stallions).
While horses aren't generally territorial, protection of food and other valued items does occur in domestic horses and 'resource guarding' can be a common cause of aggression between captive herd members.
I suspect that there aren't many natural predators around your horses and, as they're not mares or stallions that he's turned out with, I'd take a guess it may be something to do with resource guarding.
If possible, try putting them in a bigger field or maybe do some groundwork exercises with the bully to get him looking upon you as an effective leader.
This may change the way he acts when in the field.
In-hand work, where you control his movement by asking him to move away from your touch and where you control the movement of his feet will be beneficial.