This one is a bit of a mouthful. Does your GSD eat other animals' poos? That's Coprophagia. This is not an abnormal behaviour so don't think you're underfeeding your dog. Many predatory species do this as poo is an easily digestible source of protein. We as humans find it disgusting and it can be difficult to cure.
Why is my GSD eating poo?
Your GSD might just be curious and want to know what it tastes like.
Your GSD might be bored of his/her diet. If he/she is on a competely dry diet why not gradually change over to nature diet? Or introduce raw?
If your dog eats poo while he/she is on a walk, try feeding him/her 45 minutes before a walk; he/she may just be hungry.
If it's none of these, it could be a vitamin B or K deficiency.
If your GSD likes the taste of poo, spray it with bitter apple or something else that is unpleasant. You will have to spray it on every poo he/she has access to or he/she will work out which poo is okay to eat!
It might be because when he/she first ate poo you made such a big fuss out of it that now he/she thinks it's really valuable and they must get to it before you do!
If you have told him/her off for pooing in the wrong place he/she may now associate pooing with punishment so will eat the poo and other animals' poo in an attempt to hide it from you and avoid punishment.
When a pup, the breeder's house might be boring and their poo may be the most interesting thing there. Some pups might just be following the cleaning behaviour of their mother.