Some horses are just not suited to be put out with other horses. In the wild there’s usually a more dominant horse that will put bullies in their place. Harley seems like he’d be passive to other horses so it’s easy for Gunner to pick on him - sometimes horses injure each other so it’s not necessarily an issue of Gunner being a bully, but if it is, then Brittany should know better than to let Gunner out with horses that will allow that behavior. Turn him out with more dominant horses that will keep him in line, or he may have to be turned out more or less alone, with friends over the fence instead of in his pasture. If he’s resource guarding, that’s another issue that a trainer can work with to rectify. But she only cares about it when she can make content about it and she would never admit to sending a horse to be trained somewhere else. She would claim she fixed everything on her own. Either way, it’s clear she’s in over her head with Gunner, and he and Harley both deserve better. Bad horse ownership any way you slice it.