Her wearing a halter is not the fix, for you maybe but not the horse. She sees the stall as a trap so for now why not continue to feed her outside and set out 5 piles for two horses and at least 20' apart. The gelding may chase her but he'll give up by about the 4th pile. He'll try to guard them all until he realizes the futility and will settle down. If the horses weren't hungry this wouldn't happen and if the mare has ulcers she needs hay 24/7. To run out you will be fighting a losing battle. If you feed her where she's ok she will come to look forward to your arrival and that's what you want. After a few days, when she's eating, rub you hand on her neck and shoulder then back up a few steps then turn and walk away. Do a little more each time until she is accepting then start carrying her halter and lead so she can see it. Don't try to catch her just yet. If you get in a hurry to do things with her she will sense this and there will be little progress. When she is good about being caught walk her around then into the barn, stop at the stall let her look then take her back outside. Each time you stop at the stall offer her a treat or a handful of hay. You need to take your time to teach her that the stall is ok so it may take a week.