Absolutely! It doesn't matter how many gears it has. You can exercise on a 1 speed or a 15 speed or a 33 speed if you feel like it. The number of gears is totally irrelevant. Many modern internal geared bikes only have 3-7 gears and you can get around just fine on them.

The goal of exercise is different for everyone, but it generally involves getting your heart rate elevated for 15-20 minutes or more.

If you ride around the block for 5 minutes, you won't get much from it.

If you ride up hills and things like that for 15 minutes, you will get a little good from it.

If you ride up hills and work really hard at it, monitoring your heart rate and tracking your progress, getting your heart rate up for 40 minutes to an hour, you will find your fitness level rapidly improving.

If you are inclined to do more than that, you will get even better fitness improvements.

The key for a bicycle is to make sure that all the important parts are well maintained and work properly.

Do yourself a favor and educate yourself a bit about your bike, then take it in for a tune up to your local bike shop. Know what needs some fixing before you go.

Things to check:

Wheels - check that the tires are in good condition and have some tread. If you can see the cloth fibers in the tire, it's time to replace. If you can see cracks on the sidewalls, new tires would be a good idea. Try to replace your tires with mid-range quality as this can have a HUGE impact on the pleasure of riding. If you are in town on smooth roads, try a slick tire. If there are a lot of grates around where you ride, use a fat tire in the front that can handle some good pressure (65 psi or more), but isn't too heavy. That will give you a lot more stability and safety at speed.

Also check your spokes. There should be NO LOOSE SPOKES. It should be very cheap to have a local guy "true your wheels" or at least check the spokes. If spokes start breaking on you, take it in and have them replace all the spokes. It's not expensive and doing it one at a time is wasteful and could be dangerous.

Brakes - Both front and back brakes should provide significant resistance and be able to stop you fairly quickly. For cars and motorcycles, it is said that your brakes should always be able to lock up (skid), but this isn't necessarily always needed or possible with some brake types. Just make sure they give you confident stopping.

Once the brakes are done, you can check your front fork.

Front Fork - First, check that your wheels are properly seated and the bolts are sufficiently tight. The wheels should spin freely, but not wobble with side to side pressure.

Also, check the headset! This one is overlooked on almost every bicycle I have ever seen being ridden by someone who isn't really serious about maintenance. It's very easy to fix and can make a huge impact on the amount of control you have over your bike (especially for some cheaper types of fork). Simply apply your front brake and rock the bike back and forth. If you put your fingers where the fork meets the frame, you can check for movement. If the fork moves at all when you move the bike forward and backward, you need that tightened. Again, your local bike shop will be able to fix that for you in seconds, but it might require a tool that you don't have.

Now all you need to do is check the seat (it should be able to tighten so it doesn't twist back and forth), the pedals (pull side to side and check for looseness anywhere in the pedals and crank) and get your shifters tuned up so the bike shifts controllably.

The last one is a bit tricky, so get the bike shop to do it and when you do, ask if they can give your chain a quick clean and lube.

I see that you are in India, so I can't give you any price guesses, but if you find a small bike shop, I'm sure you can get all of this checked and taken care of for a very affordable sum. It will probably take less than 20 minutes to do the whole thing, depending on what needs changing. Swapping the spokes is a big job and could cost you money, but shouldn't be necessary unless the bike is fairly old.

Do this about every 6 months and you will get a LOT of use out of your bike and you will enjoy riding it MUCH more.