It honestly does not sound like that much of a problem -- horses just DO chase and threaten and kick at each other. Chances are very good that they will eventually work out the pecking order, so to speak, and settle down and get along better.
In the meantime you'd be best off respecting the hierarchy that already exists -- this means putting the mare in FIRST and feeding her FIRST. Both horses know that's the way it ought to be, and things will go a lot smoother if you don't try to disrupt their little herd structure. (You CANNOT reverse herd hierarchies by handling horses in opposite order -- it just creates chaos and resentment and problems).
Mind, she needs to be civil and safe around you. If you are unsure of her manners when both you and the gelding are around her, I'd carry a longe whip or long dressage whip with you for a while. And she should only be allowed to be haltered, brought in and fed if she is CIVILIZED about it. But, always do her first, then the gelding. After feeding, turn her out first unless she lurks at the gate waiting to 'get him'... if she does that, it's safest to put the gelding out first.
Do you by any chance have enough stalls that they don't have to be in adjacent ones? Or if they do, put their feed at opposite corners of opposite sides of their stalls, so they are not right on the other side of the shared wall from each other.
Generally though things will settle down once they've got things worked out better between them -- mainly you just have to keep anybody from getting hurt til that happens.
Good luck,
Pat