Spray collars should never,ever be used for nervous behaviour. If the GSD is nervous in any way,shape or form, then don't use it. To be perfectly honest, if you are wanting to stop the deveoplment of aggression, you should probably seek the help of a qualified behaviourist. Ask at your vets for a recommendation.
Good luck.

They are good for jumping up in play etc with a non nervous dog when all else fails, but I really think it might make things worse if it is fear barking.

We saw a behaviourist for Loki's aggression towards other dogs. The upshot was that the behaviourist thought Loki was very dominant so no fear issues.

Anyway after observing him with a stooge dog he tried a citronella spray collar in order to try to dissuade him from his aggressive reaction. Result - initial, limited success but Loki eventually ignored the collar and went about his business as usual. I.e after Loki figured it out it had no effect whatsoever.

Guess my lad just likes a fight.