To gain weight you need to consume more calories. I suggest eating a little bit more each meal. Even if you ate just 50 more calories for each meal then your calories per day would go from 1700 - 1800 to 1950 - 2100. To increase how much you eat per meal without having to change your diet too much you can either increase your portions or add some food to the meal (or both). You can easily add some calories to your meals by eating something like a piece of low fat string cheese or a handful of granola.

Umbral_Seraph suggested nuts which are good because they are a very concentrated source of calories as is both fruit/vegetable juice and dried fruit. Those are good ways to sneak in calories. Someone trying to lose weight would want to avoid foods like that and eat things with less calories per ounce (such as fresh fruits and vegetables).