When Nooki was 4 months to about 6 months, she used to be shy and scared. When a dog barked, she would run behind us and hide. If visitors came, she hid in the crate. I wanted her to get some protection training to get bold. However, few friends recommended to wait. They get mature and understanding after they turn about a year old.
Nooki is now 1 year old and if a dog barks at us, she growls and tries to take em out. If we take her to offleash parks and some dog messes with her, she will not shy or backout. She shows agressiveness & she can jump and try to tear the other dogs year. Few months back, we didnt think this could happen. However, she is SUPER friendly with humans and dogs (especially smaller dogs).
My Point: Wait and give your GSD some time. They will have natural protection instinct once they mature (12-18 month timeframe). They will get tough & they will start sensing fear and protection instincts are going to be natural. Dont invest too much till they mature a bit more. You may realize you may not even need it.