The weight gain after hysterectomy, particularly around the waist, is result of hormonal changes induced in your body. If ovaries are also removed during hysterectomy (i.e. a complete hysterectomy), it leads to an artificial menopause.
The ovaries normally produce the female hormones estrogen, progesterone, and androstenedione. Upon their removal, levels of these hormones drop dramatically. An imbalance between the levels of androgens (male hormone) and estrogen comes into play. Besides, there is a slowing of metabolism and and water retention.
If you are on some hormones, you may ask your doctor to customize your hormonal treatment. You can also try a regular exercise regimen and minimize carbohydrate intake and increase protein intake. You will be able to notice any changes only after a certain time.
If these measures prove unhelpful, you may probably try consulting a weight-loss expert and a nutritionist.