While riding your bike is a great leg workout and yes it works out other areas as well, but I don't think it replaces going to the gym by any means. But it's whatever works for you.

Oh, and just to add to my original post, I'm trying to gain weight so I've been doing the www.livestrong.com daily plate where they tell you have many calories you have to eat in order to gain a certain lbs per week. I've been doing 3600 calories a day to gain 2lbs a week. I'm started at 149 about 2 weeks ago and I've gained a little over 3lbs so far. But 3600 calories per day is a lot for me especially because I have a busy schedule throughout the day so I have to try to make time to eat. I need to be making more time to ride my bike though, but the weather has been **** lately.