My mare used to be the same way...I dont know what happened to her now...but shes a total sweetheart to our gelding...but she also pisses around him alot I think she thinks hes a stud

But, she used to be the same way to all horses...except her baby Sweetie...which even after we weaned her we put her back in afterwards and they were best friends until we lost her last winter...and now Tilly is nice to everyone...its weird...but that dosnt help you...so sorry

Heres my input
I would separate them if at all possible. Having them pastured together and having them corraled(sp?) together is totally different. Is she mainly more aggressive when its feeding time? If thats the case then I would tie her up and give her meal to her while she is tied...or else your going to end up with a skinny gelding and a very FAT mare.
But, I think I would try to just separate them completly. Can you put up a fence down the middle of the corral so each side has access to the shelter? Or put the gelding in with your other horses and keep her seperate?