Potatoes when you cook them in a healthy way such as steaming, boiling, roasting or baking potatoes will not cause you to gain weight that you're trying to lose.
If you add things such as cheese, cream cheese, dips or butter or you deep fry the potatoes then that could make you gain weight due to the high fat content.
But potatoes alone as long as they're healthily cooked and prepared should not make you gain weight.