You've already gotten great advice on the resource guarding issue, but I thought I'd jump in and add one more observation. I've never had a problem with resource guarding because I NEVER interrupt a dog when he's eating except when I have something of high value to add. So, 99.9% of the time the dog eats in peace. But I like to add table scraps to my dog's food, and sometimes it happens that I find something to add after I've set their bowl down. So I tell them to lookout, so the additional food doesn't land on their head, add the food and walk away. This is not because I'm trying to teach something, it's just life. But from the dog's perspective, it's always worthwhile! When she has something I need to see, or has found something to eat that I need to see before she's allowed to eat it, I always call her to me and tell her to drop it, I don't reach in and take it. As Sabis mom pointed out, it's much less confrontational!
On the air snapping and back talk question I'd say it depends. My puppy did and does this frequently, but not when I am telling her NO. If you're pup is listening when you say NO, but protesting a bit too, I'd say it's not a problem. With my puppy the most challenging phase in her development came at just about the age of your puppy, between 4 and 6 months. This is when she'd look right at me and choose to ignore commands. Your reaction to this is important. With almost any approach to dog training I've been exposed to over the years, you'll hear that you should be fair, firm, and consistent. With GSDs, that first element, fair, is especially important! Much more than other breeds I've worked with, GSDs have a sort of heightened sense of fairness, for lack of a better way to say it. So the first thing I'd suggest is that you look objectively at what you're doing, making sure that you are being fair, and clear. IME a GSD will protest, sometimes vehemently, if they think you're being unfair. They'll also let you know if you're not being clear in a similar fashion. So if your puppy is protesting, first make sure that he isn't right by objectively examining your approach. With my puppy I can count on my fingers how many times I've had to tell her NO, and she's now 14 months old! That being said, the other thing you don't want to do is allow your puppy to ignore your command. If, or should I say when, he does this, calmly but firmly enforce the command. Again with my puppy I use a no, which is said quietly as a sort of steering indicator, and a NO, which is a non-negotiable COMMAND meaning stop whatever you're doing immediately! This latter is used very sparingly, but when used it is used very forcefully, and no back talk is permitted! With the steering no, my puppy will often let me know what she thinks, and I'm grateful for the feedback! I hope this helps! Great looking puppy, BTW!
It's not the size of the dog in the fight, it's the size of the fight in the dog. Mark Twain