I would not trust the calculator you referenced in the question. It merely does an average calories count and has little to do with your fitness level.
The notions of calories in calories out relative to weight loss or gain is very simplistic and had been proven wrong already. See the well referenced Harvard Study in 2011 and other studies here:
Studies on Calorie Count Myths
To objectively determine how fit you are, there are very good measures that you can use. The best one is a VO2 Max test.
Read more on it here: Wikipedia Article on VO2 max
Many universities have physiology or human performance labs, and they often offer these tests for the public at very reasonable to no cost. Do a search on V02 Max lab/university & you will find plenty, all over the world. Besides VO2 tests, the labs normally can run a series of other excellent and highly accurate tests from metabolic rate to blood levels that you should definitely try if you are serious about fitness. These would give you a very clear picture of where you are, and then come back in 6 months or a year to see how you progress.
Short of that, any modern fitness machine such as a treadmill or an elliptical machine that has a heart rate sensor should also be able to give you a reasonably useful fitness test that you can do in about 10-15 minutes. What these tests usually entail are your levels of effort against the resistance changes of the machine, calculated against your change in heart rate through the process along with your basic age, weight inputs.
Wearable fitness bands with optical heart rate sensors costing no more than USD $50 nowadays can also give you some very good profile of your fitness and can be used for long term fitness tracking (I use one for training and overall fitness & sleep tracking).
Back to the specifics of your question, you mentioned wanting to gain weight and that you have to sit in front of the computer for 8 hours.
I suppose you want to gain muscles, and not fat since fat gain is as easy as doing less and eating more bad-for-you food.
Gaining muscle mass, on a functional strength level (meaning looking solidly good and feeling agile but not wanting to be a huge competitive weight lifter body) involve building up resistant (weight or body-weight) exercises to challenge your body to "grow." This would require that you either choose an assisted weight regiment or a body-weight regiment that force muscle tear, metabolic challenge at a level much more intense but shorter per round than your brisk walk (which is basal aerobic in nature). In this answer, it may be too much to go into details. However, know that you'd need resistance training for weight gain, and that you can use weights or your body weight, but they have to be proper and good movements with sufficient challenge to your body to induce growth. Proper nutrition, hydration, sleep, work-rest ratio, also come into play. I hope these ideas give you a good pointer to start looking further.
As for prolonged sitting, it's bad and irreversible. You can read this article below for a summary of recent research, but more has been known since as well. When you sit for a long period, your hormones begin to change, and adding it up through the years will take an irreversible toll on your well being. Not only that, staying in doors and looking at the computer all day also takes a toll on your brain, eyes (also part of your brain), and creativity, not to mention mood. I used to work the same way, then changed. My life changed like it dialed back the clock over 10 years. Article:Is Sitting a Lethal Activity