Is a 21 speed bike good for resistance training and cardio?

I don't know much about speed gears and how exactly they benefit...I go to the gym every other day but on the day I don't go, I pig out. I just wanna get rid of the extra flab. Does a 21 speed bike work similar to one of those bicycle machines at the gym?

Oh, by the way the bike I'm looking at is one of those fold-up bikes it's a Dahon... the model came out last year of 08 so its kinda new...

Folding bikes are NOT made for exercise. they are made primarily for getting from A to B and back to A. NOT for getting up to speed and taking out for your intended use for long rides. I sell Dahon bikes and although well built you should check to make sure the one you are looking at is not one that was recalled this year due to clamps that fail! You would be better served with a Hybrid bike of the same gearing for loosing weight as the Dahon will not likely last long for the use you intend. Folding bikes are made for leisurely rides only!
A hybrid sound just right for you.

Actually, just about any bicycle, from a single speed beach cruiser to a 27 speed road bike, will let you pedal your way into condition. All depends on you using it..riding hard for 30 minutes a day, or slow and easy for a couple of hours a day, or at least every other day.

With a little luck, you’ll get hooked on cycling and find it a very enjoyable and rewarding sport.

As for the folding bike, that would probably be your worst choice, unless you really need one due to lack of storage space. But it will get you in shape if you ride it regularly. BTW, I don’t know about the quality of a Dahon, but I do know that the Bike Friday brand is used by lots of people for extended loaded touring. Tough bike.