So you might gain a kilo or two. So what? Don't make a meal of it - the benefits will be worth it, and if you do put weight on there's no reason why can't lose it easily and gradually, as you get used to a smoke-free life.
There are no rules set in stone when it comes to giving up and your weight: a third of ex-smokers don't put on a single ounce when they stop, and for those who do it's only a matter of a few pounds.
If you do end up with a few extra pounds, isn't it normal? Smokers on average are lighter than they would be if they didn't smoke, so don't beat yourself up about having a normal weight. To find out if you're underweight, work out your BMI (Body Mass Index) by dividing your weight by your height squared. If your BMI is under 22, gaining a little weight will do you no harm at all.
Gaining 3 kilos is not the same as gaining 8. You won't see a different you in the mirror, you won't go up a dress size, and above all, you'll at least be healthy on the inside!