It depends on what kind of game you are playing with the dog. What is your temperment when you play with the dog and vice versa. Never look straight into the dog's eyes while playing; if you look at a dog's eye or any animal's eye for that matter it means you are ready for a fight. Play games like fetch. Dont play games which will irritate the dog like pulling its leg. Any game should be a two way enjoyment where both you and your dog enjoy; it shouldn't be something like only you enjoy and your dog doesn't. A dog can't express it's feelings as to whether it is enjoying the game or not. So we need to make sure by the way it reacts that it is enjoying the game.
There is something called as 'Alpha' in dogs who is the head. In your case probably the dog thinks that it is the 'Alpha' and is biting you to tell you that it doesnt like the way you are playing with it or there are some dogs who play by biting, it is probably biting you in a way which doesnt hurt you. This also happens if a pup is taken away from its litter at a very young age.
I think that the dog can be made not to bite by just giving the command "No" with a firm voice (when it tries to bite) and not playing with the dog for 5 minutes and then again go and play and repeat the command when it tries to bite.
GSD's are gaurding dogs. In your case, the dog is already 6 months old and it considers you a Stranger. Are you visiting the dog everyday? If you want to be a friend to it, visit it everyday. As Hardyboy said, your friend should socialize his dog properly unless and untill he uses the dog for gaurding purpose
I came across this site which might help you: http://www.wikihow.com/Play-wi th-Your-Dog