Umm... Why do you need the spell?
I don't know of one to help you gain weight but you could try writing your own, be sure to state your goal weight and visualize you slowly gaining the weight. No spell will make you loose/gain a large amount of weight over night.
I suggest you speak with your doctor first. If you are underweight it could be a medical issue. They could also recommend a diet to help you gain weight safely. Don't grab junk food and stuff yourself. Try eating more healthy fats, whole milk and dairy products, and high protein meals coupled with weight training to gain muscle mass. Spells to gain/loose weight can be a great motivator, but you need to do the work.