Eating plain salads should not cause any weight gain because salads are pretty low calorie when it comes to food.
If you added lots of meat such as chicken, pork, bacon etc to the salads then eating a lot of those salads may cause weight gain.
Salads especially those salads without all the meats are healthy for you and one of the healthiest foods that you can eat.
Salads are also one of the best low calorie foods that you can eat when losing weight.
The salad can help you feel full so that you don't eat other higher calorie foods so you lose weight.
Some people have lost weight by eating only salads for a year.
Exercise also helps along with staying active.
For example a side salad contains around 15 calories and at most a full salad contains around 50 to 60 calories which is not all that much.