If you're eating the right foods, you're body will have specific instructions to not put on excess weight, even if you spend most of the day sitting. As long as you are getting up every so often to move ur lymph, keeping good posture, and sweating every so often, sitting for most of the day shouldn't impact weight gain in healthy people. Obviously if you're body is running codes which lead to storing fat, you will need more movement to counter these codes.