Weight gain is seriously about calories in versus calories out. If you aren't eating well, you will lose weight. It is that simple.
The same as if someone is trying to lose weight, you should keep a food journal and write down everything you eat. You need 3500 calories to put on a pound. Not by eating junk though, that kind of weight you don't want. But you will need to work on eating more.
Low fat weight gainer will not make you lose weight - unless that is all you are eating and you use it as a meal replacement and not eating anything else. If you add that to an already healthy diet, you would gain weight due to the caloric load.
The smoking is bothersome since you are actively trying to become pregnant while smoking. Smoking is terrible for you - let alone a developing fetus. You should quit that first of all. This will also add weight, since your body's metabolism is sped up by the cigarettes.