Well, the first time my GSD met a cat was the first day I found him, I was trying to leave him out in the yard but he slipped in the door under me and went to chase the cat who was sitting in the middle of the floor as I watched in horror but couldn't move quick enough to grab the dog and of course he had no training yet. It played out almost in slow motion, the cat stayed perfectly still until the dog reached out to grab him and then slashed the dog's face with his claws. Poor Hector still has a little scar from it but he leaves cats alone now.
Don't do it that way, please.
I've introduced a lot of dogs and cats because we do some fostering for rescues sometimes, and we have 4 cats. Here's what I do:
1. Leave them separated for a couple of days, so they can both smell each other (under a door or whatever) but not have to interact. Once everyone is relaxed and ignoring each other, move on.
2. Let them see each other through a barrier. We have a "cat room" in a spare bedroom and when we get to this stage, we start by putting up several baby gates stacked so the cats can't jump out. Letting the cats out when the dog is in a crate would also work. While you're doing this, work on your "leave it" and "watch me" type commands. Reward your dog for ignoring the cats, especially if they do something interesting. However, I do let the dog sniff the cats if the cats are up near the barrier--a short sniff, then a command to redirect their attention.
3. Once seeing the cats through a barrier isn't a big deal, start letting them out while keeping the dog leashed to you. Again, reward the dog for ignoring the cats. Also, make sure you don't let the cats torment the dog too much. What I mean is, some cats will come up and climb on the dogs or play with their tails or whatever--don't let this happen. Your dog can't get away from sharp claws or uncomfortable situations when he is leashed, so err on the side of caution.
I start this phase by only going maybe 20-30 minutes at a time, and actively training the dog during that time (just obedience stuff, but keeping his attention on me). Gradually we both extend the time and allow the dog to just relax. When we get to where we can sit and watch a movie with the cats in the room and the dog ignoring them, we'll start removing the leash.
How long this takes depends on the dog, and make sure the dog does see the cats run and play a bit (and ignores them doing so) before you start letting them loose together. You don't want to think your dog is okay with cats but then have a zoomy cat trigger his prey drive and get a bad result.
Also, I always make sure the cats have a refuge--I mentioned the cat room we have, and we always have a baby gate there so the cats can jump in and get away from the dogs if they want. Vertical space is also good for this, if they have a tall cat tree or something of that nature.
I also don't leave dogs and cats unsupervised together for quite some time. Again, the fear is that even though they're normally okay, something could trigger the dog's prey drive and without you there to call them off, it could be disastrous.
Also, throughout the whole process you should be reinforcing your dog's recall and leave it commands, just in case something goes wrong.
That's probably more detailed than is necessary for a lot of dogs, but I've introduced dozens of dogs to cats over the years and I've never had a bad result doing it that way. I would say that for the average dog with a decent prey drive (I foster herding breeds, so they can be pretty chasey) you should anticipate taking 2-4 weeks doing this, but some dogs may take longer. I wouldn't expect less than 2 weeks, even if your dog is doing well--take your time just to ensure good results. Just don't rush it and teach your dog that cats are absolutely off limits, and things should be fine. The nice thing about GSDs is that while they do have a strong prey drive, they are also a herding breed and seem pretty responsive to discerning what species they can chase and what is of limits.