I think if she is doing good on grass and hay alone (especially if it's nice lush green grass)....then grain is not of importance. Grain, correct me if I am wrong, is for horses who need a source of concentrated energy for one reason or the other (underweight, working, senior, ect) ...
I'm sure she probably doesn't need too much grain, if any, if she is supported fine off pasture and hay (especially if you say she is gaining weight).
However, if you still want to grain her, you can just give her a little (like for a treat), or you can get her a ration balancer type grain (which is meant to be fed in very small quantities)...or a grain that is lighter in 'energy'....such as a weight control or light formula. I think the ration balancer is a good idea, to balance out the hay and grass.
I hope this helps!