If you eat a lot of cabbage the cabbage could make you gain weight.
However eating just a little bit of cabbage will not make you gain weight because the cabbage is a low calorie food and that makes cabbage actually great for weight loss.
When you're on a weight loss diet then eating cabbage and even cabbage soup helps you lose weight.
The cabbage being low in calories and fat content means that you can eat enough cabbage or cabbage soup to help you keep your stomach full.
But you then lose weight because you also do not want to eat anything else.
To lose weight you need to eat less calories and burn off other calories and fat.
So by eating cabbage and cabbage soup that is low in calories and fat content then you need to burn less of those calories.