I think the worst thing about gym is that I didn't learn anything there.
I didn't learn about calories, or healthy eating. I didn't learn about basic cardio, why it's good for you, or how often to do it. I didn't learn about how to build muscle, what reps or sets were.
Gym amounts to mindless busywork. Do stuff or you get an F. It could serve as a foundation of knowledge to set yourself up for healthy habits for the rest of your life. Instead, it has the opposite effect of training you to hate fitness.