You can rub their coat down with apple cider vinegar. But for the most part GSDs aren't a smelly breed. As long as their diet is good and they are healthy, they tend to not smell. The only time my dog gets an odor is when he doesn't dry properly after swimming.
ADCH Mikko (USDAA) SCH-B; (NADAC) EAC, EJC, TN-E, TG-E, VerO; (AKC) NAJ, CGC; (NACSW) NW1 - 9 year old GSD