Pain in the gums can be due to lot of reasons but the most common ones are related to decayed teeth in that area as the roots of the teeth extend inside the gums. A radiograph reveals a clearer picture of the cause. Your dentist may have identified the exact reason for the pain to be the tooth in question. You can go ahead with the treatment. If still you have doubts or not clear with the line treatment, you can always take a second opinion.