and welcome to the HF (horse forum). You will find great information here!
For weight gain, I would recommend beet pulp. (i prefer pelleted but shredded works too). Our horses are on triple crown light and beet pulp (although they do not need to gain weight, they are at a perfect size). Beet pulp also keeps the horses weight balanced.
If I were you I would look into beet pulp.
P.S. - Lets see some pictures of her!