Graves diseases is caused by an over active thyroid. The increased thyroid hormone causes an increased metabolism leading to weight loss and sweating.
Methimazole works on the thyroid to decrease that function. Since your body has been put on overdrive in metabolism by the thyroid hormone excess, its playing catch-up. So that includes a weight gain.
The weight gain shouldnt be that excessive. Ask your doctor what would be your ideal body weight.
If you have Graves disease you must have lost a lot of weight before you were diagnosed and treated. If you have gained too much or find that you are unable to function because you are on low energy, you need to see your endocrinologist and have your levels tested. You probably need a dosage adjustment.