If only they had the chance to settle their hierarchy in a normal herd environment, it would pass rather quickly. Otherwise, as they are paddocked in seperate paddocks most of the time, the gelding is probably seeing his paddock as his territory and just protecting it from the "stranger".
Horses are herd animals, they absolutely need other horses to live with in order to stay psychologically completely sound. But they also need to be provided with the possibility to live within a herd, not just see other horses over the fence. Then aggressive behaviors come quite naturally...
I have come a long way, to surrender my shadow to the shadow of my horse.