You mentioning the gate and her bolting, in addition to the fact that she was only taken from her stall to ride with her previous owner, makes me think she's a bit barn sour, and resents riding/working.
You need to start on the ground building a relationship, especially since she's so new to you. Take her out of her stall or paddock to groom her. Walk her around, let her graze, give her a hand full of grain, then turn her back out.
Purchase a lunge line (you may not even need a whip) and work on lunging and get her listening to your commands, like close2prfct said. Once a trusting, working, respectful relationship is established, she'll soon understand that working with you is a confident partnership, and not just "work."
I would also recommend lots of walk/trot work, backing, stopping, and directional changes to get her listening to you. Once all of that is solid at the walk/trot, then consider taking her up to the canter.
Best of luck. :]