I don't think gym exercising is as holistically healthful or as sustainable as getting out and doing that which you love to do, but it's better than nothing. And yeah, maybe some people LOVE the gym. Well, they need to get out more.
But whatever, I semi-kid. It is better than nothing.
Among other things, outdoor activities mean more vitamin D, which is supposedly crazy deficient in our culture. Plus the variety is something you cannot get in a gym. Yes gyms have a million machines and such. But the synthetic nature is a weak substitute for actual connecting with real nature, which, as we all know, has tremendous value not only for physical health but also for mental health. Humans are a social species. There is something very right, maybe even primal, about taking to the woods in small groups.
Also, I think people in general spend too much time in their comfort zones. Going to the gym is pretty easy. You plan your workout, you know how it's going to feel, you know the variables before you even get there. You arrive, you practice a specific pattern that rarely strays into a zone in which you need to engage your mind. (Yes, I know about focus, I'm talking about something a little more abstract than that.) I just think so much more can be gleaned from putting yourself in a place with slightly less predictability, with a little more contact with elements, and more opportunity to expand your brain and think new thoughts.
Gym in addition to outdoor stuff, cool. Gym as substitute, meh.
Yeah, I'm an idealist, but why not always try envision a better way?