I liked to use a rake and comb on my GSD, they seemed to work best. I never tried the furminator so I have no opinion on that. I did use the shed ender which worked surprisingly well, but it took some time.
I use a Furminator brush for my German Shepherd. It was recommended by my dog trainer. They're pretty expensive, but its a really good quality. Lasts for years and still going. Works amazing for dogs that have the fluffy undercoat like GSDs. Sometimes when I give her a good long brush, so much undercoat fluff gets brushed out that I could make another dog with all the fur I collect. It's crazy.
I work part-time in a grooming salon as a bather, so I deal with GSDs more then the other groomers and even though we sell a Furminator package, we do not use the Furminator tool 9 times out of 10. I recommend an undercoat rake, they work wonderfully and don't risk damaging your dog. The problem we've found with the Furminator is if you use it too much or too hard you've got a huge risk of irritating your dog's skin or even cutting it if you aren't paying enough attention. Honestly, for shed I suggest buying the Furminator conditioner and then blow-drying on low or no heat. That blows all the hair out and works better then hours and hours of brushing. I'm lucky to have an industrial dryer at work, but a regular hair dryer that's powerful works too! Hope I helped!