This being related to the inflation spell you were asking about earlier I assume, then no there won't be anything to help you.
Spells are subtle, if for instance you wanted to gain weight, then there may be an spell that can aid a (healthy) increase in size for those who struggle to put on weight, but I assume you want something far more drastic.
No spell, can make you put on 3.5 stone/22.7kg.
Spells can however aid it, my best advice would be to consult your doctor if you are worried about your weight and they can recommend the best way to safely put on weight.
On the spell front, you could adapt a spell on this site to fit your needs, or make your own that can aid what you are doing.
No, spells are not going to make you suddenly gain weight.
I will also point out that you have now posted five posts on this topic in various forums. This could be considered spamming which is against the rules of this site.
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.
Just try a richer diet instead. If you are an active person,then something only a tad bit richer in calories won't do bad to you.
You may also try eating dinner later than usually.Scientist have proven this helps yougain weight because the calories you get will not be consummed due to the fact you are going to sleep,so won"t do much physical activity.
However the second option is less healthy so I can only advise you again to not be afraid to add up that extra slice of bread or an extra bowl of cereals to your meals.
Opt for food that contains more calories but not that is very rich in fats or sugar. While they greatly improve weight,they do the same to your risk of getting a heart disease,so be careful.
I believe this may be a good way to achieve your goal.Goodluck!