Let's not beat around the bush. Yes after quitting smoking you might gain a little weight. Not much. Statistically 2-5 kg. At first.
It’s due to the fact that cigarettes are appetite suppressants, that nicotine causes an increased use of energy and that now you will increasingly allow yourself something sweet, as you’re already not smoking anymore. That’s okay too and if you can limit it and compensate with movement you will soon be back at your desired weight.
Diets are generally idiocy, a healthy lifestyle is the solution. It’s that easy. It’s not that hard, right?