Re: I need your advice on weight gain
Feed her small amounts of whatever pelleted/grained food you find is best for her many times a day. Horses are designed to eat small amounts. So if you give her 4 cups of feed, divide it into 4 feedings of 1 cup each if you can. Let her eat as much hay as she wants and make sure she has lots of water. Watch her eat to see if she may have need of floating and she may need to be wormed also.
I took in a young underweight and fed her 17% pellets until the weight gain was steady, then reduced her to 12%. But, keep in mind, I never worked her while she was so thin - just walked her in hand. Since your girl is older, you may need to add beet pulp to help her out -- it's important to keep the beet pulp feedings small and very wet until she gets the idea that she doesn't need to worry about having enough food. She may gulp her food and then risk choking. If she does eat too fast, put some LARGE stones or pieces of salt block in her feed pail to slow her down.