Finding and working with a trainer that is balanced, and has GSD-specific experience, is always good advice. But I also wanted to take a stab at answering some of your questions. First of all:
My worst fear for owing this breed just came true. I am genuinely asking for real advice and knowledge.
Breath! And lighten up!
This is a puppy, he's bound to do things on occasion that are undesirable...that's why we train /teach them LOL!
So today my girlfriend took Brutus to Academy and Atwoods. She said he was acting very aggressive towards every male that came near her or she passed. She called me and I could hear how much he was barking and believe me it was deep and meaningful. On top of that he even lunged towards someone and barked right in the face of a kid. He has never shown any aggression EVER. Why would he act this way? How do I stop this behavior immediately and how do I train him out of this?
You don't stop most behavior "immediately", so again, calm down. If you get overly upset your puppy will pick up on that. Don't be angry, and don't be embarrassed, just deal with the situation calmly but firmly. But first, ask yourself a couple questions.
Has he shown any other behaviors that seem "different" than normal, like being bothered by things he's been good with before?
This is important because puppies often go through a couple of "fear" periods growing up, where they'll be a little freaked out by things - and this can be people, other dogs, trashcans, etc. - that didn't used to bother them. My puppy, a female, did this at 8-9 months, but my understanding is that males often have them sooner, so 7 months is about right. If this is the case, it's best to keep them away from these triggers until the fear period has passed -
a couple weeks at least. Just watch closely and you'll notice him getting back to his usual self where things don't seem to set him off as much anymore. Correcting a dog for their reactivity during a fear period can have lasting negative effects, so don't...
Do you take him out as frequently now as you used to?
It's not unusual for new puppy owners to take the puppy everywhere initially, then slack off a bit as the puppy grows. Let's face it, for a number of reasons large puppies, in the 5 month to 1 yr age can be a bit trying at times, so taking them everywhere is not quite as fun as it was when they were still little and cute lol! If this is the case, don't feel bad, just acknowledge that your pup will need some additional work to get back to where he was. As others mentioned, take him out and just keep him at a comfortable distance from people until he seems okay, then very gradually move closer. Praise calmness, and gently correct reactivity. He'll come around if you don't rush it. If he's being reactive, move away a little, then praise calmness and move closer more slowly. The time to correct him is BEFORE he's barking and lunging. Remember, the "correction" is intended to help steer him toward the behavior you want to see, not a punishment.
Does your puppy go to public places with your gf without you frequently?
If not your puppy could very well be reacting this way because he's a bit unsure of the situation, and because of that he got a little overwhelmed! If this is the situation, just work on it. Take him out and about with both of you, but let her hold the leash. Gently correct any reactivity, and stay fairly close at first, increasing your distance from her and the puppy over time. Remember it's all new, give him time to experience things and adjust!
And most of all, remain calm LOL! I hope this helps in some way...Good Luck to you!