The gentle leader can be a very helpful tool, but it's very important that you introduce it as a positive thing to her. At first, she WILL try to get it off, dogs just aren't used to having something on their face like that. I'd suggest before actually walking her with it to get her accustomed to it gently and to associate it with good things. Showing it to her, giving a treat; putting it around her neck (but leaving the face/muzzle strap dangle) and treating her; and so on. When you put the muzzle strap on, let her wear it and treat her every time she's not trying to get it off. I'd do this for maybe 5 minutes at a time at first and work up, adding a leash when she seems more accustomed to it.

I think if you just put it on and attach a leash right away and try to walk her it's going to be a fight and she may immediately associate it with something unpleasant and continue to struggle any time it is on her.