Scars or other marks are most likely from play as a puppy with other GH puppies. Rocket is missing the tip of an ear and it looks like it's been that way for a long time. He has one or two other small marks as well.
As far as the growling: We have a rule in our house even after 5 years. Rocket does not get petted or touched unless he is standing. This means he is awake. If he's lying down, he can be sleeping or groggy, even with his eyes open. We've found that "no petting unless standing" is the easiest way to teach kids or visitors the rules about Sleep Startle.
I have been bitten once, and so has a dog sitter - both time our own faults. We both thought he was awake when he wasn't. One of those large canine teeth through your hand is no fun, and the Animal Control Warden showing up is also not a fun time. This is the same boy who now roaches against my legs and goes to sleep, but I still call his name and wake him up before I move. That's his version of snuggling.
You will find that over the course of 12-18 months you will have a completely different hound from the one who walked in the door. it's a lot of fun to watch the changes.
Also, just an FYI - the word "rescue" is often a heated topic. When people ask if Rocket was rescued, I usually whisper and say "He's adopted, but we haven't told him yet." That diffuses the question most of the time.