When you say "hard mouth", would you mind describing exactly what she was doing and what was going on? This would help clarify things a bit. Sometimes dogs "stick", which is different from "hardmouth". Hardmouth generally describes when a dog actually chomps down on the bird and crushes the bones to were the game is not fit for the table. Sticking is when a dog will not release/give/drop the bird when commanded to do so. This can result from to much pressure, playing tug of war, something could have happend when she was little pup that may have caused the sticking...however it can be cured by finding a good GSP FF book, looking up on youtube (some good videos with GSP's that show the do's and don'ts about FF).
The key to FF and curing sticking or hardmouth is consistency and repitition. Teaching hold, not with a frozen bird, but using your hand with a leather glove on and teaching hold that way, this helps from the dog chewing and chomping down.
Have your dog up on a table/elevated surface at eye level...if not then you will have to bend down to the ground. Initially you will insert your gloved hand into the dogs mouth and command "HOLD". When she chews, try to spit it out, grab the rough of your dogs neck, and pull up/back. This should stop her from chewing or spitting all the while you are still commanding HOLD. Once she accepts your hand, tap on the underside of her muzzle/jaw and command HOLD. When you go to take your gloved hand out of her mouth command give/drop/release (whichever command you want to use).
Do this everyday for no more than 10minutes. Keep it short and afterwards to relieve the stress, throw a happy ball or something. Always ends on a good note as well.
I would still recommend checking out videos on youtube and there are some decent ones out there that give a better idea of what I am describing.
I hope this helps and let me know if you have any questions or PM me and I can give you my number.
Greg