If the bolting is after she is through the gate, I would ask if another horse in the field is or has been bullying her. If that is the case, putting her in first should solve it easily. One of my mares would bolt when I first got her (so she could go to her best pal), and I used the stand and wait for a treat method. Worked within days – haven’t ever had an issue with her since. You may need the cooperation of one of the people who does regularly turn her out – have that person with you, bring the mare through the gate and turn her to face it (give a treat when she does this). Require her to stand that way while you remove the lead or halter, whichever you take off – give a treat. Progress so that she will eventually stand and wait for her treat until you (or the other person) are on the other side of the gate and it is closed. If she is off the track, tell her to “hold back” – she should know what that means.
It is never the horse's fault