And now my 2 cents. I also agree with everything Mary said above - and everything A & C said, except, that part about GSs being spooky. I've had GSs for 30 yrs and that's just not true - if it were, they'couldn't be used for Seeing Eye, K9/Police work, Protection dogs, etc. They're the most widely used dog in the world. Yes - they've been bred to guard, and they take it very seriously. If you're looking at a dog who already has "issues" and you have no experience, you may want to look at another. Any reputable GS rescue (and if you need one, I can help you and find you the perfect dog if you're in the US) will will be able to give you assistance.
Many years ago I took in a 15 mo old female who had not been socialized at all. I knew her whole history, she had been kept in a backyard in a 6x6 kennel, and possibly abused by a man. She quickly made the decision no one was coming near me, esp strange men, and even ones she had met. It took me a bit to realize why neighbors crossed the street or gave us such a wide berth while walking until someone told me - she was quietly baring teeth, then suddenly lunged out and bit someone one day. Then again. And again. Thankfully no damage done (meaning, skin broken - only clothes ripped). I was scared to death of my own dog - not for me, but terrifed to take her out. I found an incredible trainer, and altho it took quite a while, we started at the beginning with her, socialized her, started obedience training with her, and over time, she became an incredible dog - stable, trustworthy, etc. After probably the age of 3-4 (hard to remember now) I never worried about her again - she loved men, women, kids, other dogs, etc. and I could take her absolutly anywhere. Yet - she has been the most protective (and wisely so) dog I've ever been around. Her sense of who she could trust and who she couldn't was amazing. Was she "spooky"? Absolutely not. She needed to understand that she didn't need to guard and protect and be "on" (to an extreme) all the time. Once she relaxed, gained socializationa and confidence, got out in the world, learned the ways of the world, etc, she gained that understanding and it all fell into place for her.
Without knowing anything about this dog, or your level of training abilities, family, kids, etc, it would be hard to guide you. If you have a busy household, I'd perhaps dissuade you only for this dog. I think when they're learning who and when they should protect and guard, and when they can relax and enjoy, they need some "down time" and in a busy household where lots of people are always coming/going, it may be confusing to the dog for a while. If you house is quiet, and you're set on this dog, then I'd try to first find a trainer you're comfortable with - and there again, is something to consider. You don't want anyone aversive, no yanking, jerking, old school choke, prong, or shock collars (and with GS, the first thing many trainers want to do is put a prong collar on them - as "they can handle it") - as that can make things way worse.
Lots to think about, I know. But you're thinking about them at a good time - before you make the decision, rather than after - then saying "oh nooooo, now what?". Good for you.