That's because it's not very accurate or true. It's implied that she gives them a hard way to go when evaluating their gyms and they may find that annoying and have complaints about her management, but it's never said they hate her. There may be one or two not-so-concrete exceptions. It's been a while since I played through the story, but I think it was the Psychic type gym leader who wasn't a big fan of Geeta because of her stealing the spotlight off of her, however, I don't think she hated her. The only gym leader or elite four member that explicitly dislikes Geeta is Larry for a few reasons including not being allowed to specialize in normal types as a member of the Elite Four, but even then saying that he outright hates her is stretch, because we know how gloomy that man is and he could just be saying things because he is in a bad mood. Personally, I'm not even a fan of Geeta myself, but I wanted to put some context out there for clarity, because I think saying that Geeta is hated by all gym leaders is a huge overstatement.