First of all, rice bran or beat pulp aren't really ideal for true weight gain. They are fillers. It's a great way to give a horse fiber, but the calory value of beat pulp is extremely low. Which makes them ideal for maintaining weight of a horse, and it will EVENTUALLY cause some weight gain-but it'll be extremely slow if just fed by itself.
But if you truly want your horse to gain weight (as in more then about 50lbs or so), choose a product that has a high protein level, add some oil and then beat pulp (or rice bran, although I like beat pulp better) in addition to that.