Definitely sounds like you want to get started as soon as possible with some training to help with the problem. Most likely this is partially genetic, since many GSDs are prone to suspicion/fear toward outsiders. The first step it to go through the self-help section and make sure you are following all the “pack structure” rules to be sure you are in a position to start guiding her. From there we can give you some suggestions. Also, has she been to any puppy socialization classes? The more you can get her around people right now the better. You are correct and thinking that this can escalate to a fear biting situation.