OP, you're asking a simple question that has no simple answer. IMHO, there are very few dogs which one could say naturally acts, behaves and works as a guard dog...also what exactly do you mean by "guard dog?" One man's guard dog, may be considered a poorly-trained, junkyard dog by another's standards.
I don't consider the GSD, or any of the police-style herding breeds (Malinois, Dutch Shepherds, ect.) to be natural guard dogs....their working lines (and even some show-oriented lines) will exhibit protective and guarding instincts with little to no training. But without proper training and socializing, those instincts can lead to big problems for the dog and you, the owner. Also, being of the herding type, many of these dogs will have some amount of prey drive, if not a lot of it, which again, without proper training, can lead to behavioral problems. I consider the GSD, and Malinois and DS to be working dogs; meaning with the right training and upbringing they can be used for a variety of functional jobs (police work, SAR, herding, agility sports, hunting, general farm dog).
But it is unrealistic to expect a GSD to become a "guard dog" without some amount of training. And again, what do you mean by "guard dog?" If all you want is a dog that will sit around your property watching out for hostile critters (both 4-legged and 2-legged types) there are other breeds within the LGD (livestock guardian dog) category which are more naturally inclined to guard and protect in that manner. A few off the top of my head: Caucasian Ovcharka, Great Pyrenees, Anatolian Shepherd, Mareema, Tibetan Mastiff, Akbash. These types of dogs generally operate off of a "defensive" drive rather than a prey drive when they "guard" property or family.
Edit: Training and socializing is still highly recommended for these breeds as well. But due to their historical employment as livestock guardians, the "guarding" instinct is very easy to bring out of them.