How long has it been? Time is going to be the most important factor here. Your cat’s world has just been changed in a big way, and now you have to give her time to adjust.
Make sure you have at least one large cat tower for your cat. She needs to have a place she can get to that the dog can’t get near her, and generally they feel more secure when they can be high up and surveying things from above as opposed to hiding behind the couch or under a bed. The more places you can give her to get up off the floor, the better.
Baby gates with the small little opening that can block the dog out of an area but still allow the cat to pass through are also helpful. Babygates are also nice because they allow the cat and dog to see and smell each other, but keep them physically separate.
MOST cats will adjust given time and good pet management, but a few never adjust. Although the chances are small, you need to be prepared that this may not work out. What will you do then? You had the cat first, so your first priority needs to be to her. So if she never accepts the puppy, (and GSDs are BIG dogs, especially when you are a small cat), are you prepared to keep the two separate for the rest of their lives? Are you willing to rehome the puppy if it comes to that? It probably won’t come to that, but you do need to keep the thought in the back of your mind.
Your dog and your cat may never be best friends, but you do want them to at least tolerate/ignore each other. Our cat was a foster failure. We already had an adopted large-breed dog when we brought her into the home as a foster. Our dog, even though he’s about 70 pounds to her 10, is actually somewhat afraid of the cats, (like, he’s a little curious, mostly ignores them, but if they hiss, he immediately backs off! lol!). Despite this, she was afraid of him. She’d been a stray, so she wasn’t comfortable with dogs, sudden movements, loud noises, etc.
It’s been a slow process, but she’s accepted that he is part of the family, too. I’d say it took a few good months before she was comfortable enough not to bolt if he got too close. Now, she’ll still sit on my lap while he comes to the edge of the couch and puts his head right next to her for me to pet him. She’ll even sniff him, and walk near him. She still runs when he comes thundering up the stairs after beind outside, but other than that, (he has never tried to chase her, mind you), she is no longer afraid of him. I doubt they will ever snuggle up with each other; I think I would faint if they did! But if she hadn’t been able to get over her fear/hesitation around him, we would never have adopted her, because it would not have been fair to her to force her to live with an animal that made her so uncomfortable.
Hang in there, though, and give your cat some time to adjust before you panic. Watch some episodes of “My Cat From Hell” with Jackson Galaxy so you can learn a little bit about cat behavior and how to make your home the kind of environment that will help her feel safe and secure. Good luck!