Can growth hormone help a 21 yo with malnutrition in puberty grow ? how effective ? is benefits weighed more than risk?

Probably not: If your growth plates have fused (and at 21 they most likely have), then growth hormone will not provide any growth at all. It can have serious side effects such as promoting the growth of cancer cells and is very expensive, so it is not an option in your situation. You have likely hit your final adult height at this point.

No: A 21 year old has likely stopped growing. Malnutrition and subsequent stunting would not be helped by growth hormone. Growth hormone really benefits those who are growth hormone deficient. One would imagine that a person who is malnourished lacks important proteins needed to bind growth hormone, and in that case providing more would not be helpful.