I think it is good to try not to gain too much weight in the early stages of a quit but don't obsess over it. You can always lose weight later. You can't, however, lose some of the illnesses that come along with continuing to smoke.
A lot of people picked up exercise early on to keep them occupied and keep their mind off of smoking. Replacing a negative habit with a positive one is great.
I was also told early on to drink a lot of juice and water in the first few days to speed up nicotine leaving the body. Drinking a lot of water can also help keep you feeling full and less apt to snack too much.
However, please remember that quitting smoking is the healthiest thing you can do right now. Gaining a few pounds in the early days is not the end of the world. Many people who have quit have later picked up exercise and leaving an all around healthier life. Quitting smoking is a huge step towards a healthier life and can lead to other lifestyle changes down the line.