Can weight gain restore breast volume and lift sagging breast caused by weight loss?

Probably not. You should talk to your doctor and dietician about the best plan for you as every person is different. There's always the option of implants or a breast lift(depending on an in person consultation) to address your concerns!Always insist on a board certified plastic surgeon. 

Yes, I do think it could be related to your weight loss. When you lose weight, it not only reduces the size of your waist and thighs, but it might also reduce the size of your breasts – sometimes dramatically. If this happens, you might be left with saggy breasts and stretched skin. Therefore, it's possible that if you gain weight, you might gain back volume. But you've lost elasticity in your skin, which won't be affected by weight change. The droopiness might remain the same.

Yes, weight loss can affect your breasts, but was your weight loss intentional? Because having a healthy BMI is much more important from a long term health stand point than sagging breasts. Those can be corrected with a breast lift. Hope this helps, Dr. Nazarian