I don't ever do any cardio in the gym. Riding takes care of all that, and if for some reason I'm not riding, I go swimming (I have a great swimming pool right across the road).

I do go to the gym though. It's mostly for strength workouts, and, for the last few months for rehab exercises. I find that I'm more focussed on the exercises at the gym; they can be done anywhere, but I tend to get distracted if I do them at home. As for going to the gym and lifting weights... I DO need that, unless I find some sport that helps one build upper body strength and endurance. Cycling does nothing to upper body, and before I started strength workouts at the gym I couldn't do one single lousy push-up. Not that I ever need to do push-ups... I just wanted to be stronger. If you don't, you can definitely ditch the gym and get your cardio workout through cycling.