I had a long-haired mixed breed that needed frequent cleaning due urinary incontinence. I just had her stand in the tub, and used a hand-held shower-head with extra long hose. I used regular dog shampoo. You'll get more than just her privates wet, but better than doing a full bath. I also trimmed the hair underneath short to make the regular cleanings easier.
If you have a long-haired GSD, you MAY need to do what groomers call a 'sanitary clip'. Get a pair of scissors, and trim the hair around the anus and urinary opening really short to prevent soiling and irritation.
I have seen collies and shelties with the hair so matted and filthy in this area that it was a wonder the dog could eliminate properly at ALL!
The one dog had maggots around its anus - my groomer nearly quit over that one! :surprise:
Assuming that there's nothing serious going on (is there?), you can simply use dampened paper/cloth towels, as Car2ner suggests, or buy unscented, disposable baby/dog wipes. I had IWs for years and the breed is notorious for what people in the breed called "hairy" body parts. So, I quickly learned to do and maintain the sanitary clip that Sunsilver described. It's not complicated, just clip the hair around the anus, vulva and/or penis very, very short. Do make sure to buy and use scissors with blunt/rounded tips so that you don't create other problems.
ETA: If you've not done this before, consider tying her to a post or something, so that she doesn't scrabble off. Assemble ALL of your supplies before you start (before you tether her, in fact). Take a deep breath, smile, go slow, speak calmly and happily, and liberally dispense treats. She may be startled by this, but if you act like it's no big deal, she's likely to settle down quickly. The first time cleaning a dog like this, I like to stop before the dog gets too uncomfortable. Unless it's a hunking mess, you can come back in a couple of days and finish things up. That said, I'd suggest you come back in a couple of days and clean her again, anyway, cause I think it's a good idea to get dogs used to handling.
I would be careful with clipping a dog's hair it is ...how to phrase it... directional? It serves to draw urine away from the dog whereas clipping the fur could result in the urine spreading over the area.
I am not sure why you couldn't use any one of a variety of doggie or baby wipes sold in stores.