Jill is right, a grey needs to be in the house in the middle of everything to allow her to become one of the flock, she will growl but it is the only way to get her used to having you around. Talk to her softly, read a book or the newspaper to her, play with some toys in front of her and make out like you are having a good time with them to encourage her to be interested in them and she will be interested if you are having fun.
This will be a long road to travel but it will be worth it, she needs lots of time and much patience on your part and your family's too for her to ever come to trust you, she may never fully trust you since you don't know anything about her past and how she was treated but you should be able to come to some kind of understanding with her and be able to coexist in the same room without her growling. I hope you have the patience to work with her to make her a pet for it will be a tough one but she has to be inside the home with interaction with the family for this to be achieved, good luck to you and keep us informed.