1. 3.5 players can have valid opinions about 4.5 players, they can also be wrong, but the rating of an opinion giver doesn't mean the opinion is automatically wrong.
2. GSG can beat those who beat him, sure, certainly possible, conditions aren't the key to this, I don't think and neither do conditions matter that much in rec tennis (more on that below)
3. Even if I concede that conditions matter to the success of GSG, which I think he is good anywhere, serve and volley tactics work BETTER in the heat and humidity than indoors. (more on that below)
I'm going to fall into a fallacy, the all about me anecdote, but here goes: I learned tennis in Dallas, it's hot, crazy hot, I enjoy it being hot, my favorite condition for no other reason than it is fun and reminds me of old times is a 105 F real temp, heat index above 115. I've played on 4 continents, all the surfaces, in sub freezing temps, indoors with climate control every month of every year pretty much, indoors without climate control, day and night. Only on grass did I think to myself, wow, look at all these free points on my serve, this is awesome. Every other condition is so close that there is no way it would change the outcome of a match. What about Nadal and all his Spaniard friends? Well yes, those who grew up on red clay do have an advantage and a real winning one at the pro level, but how many of us in the US are playing on red clay more than once a week or month?
What about GSG? At the 4.5 level, in Atlanta, there is no way people are losing to him because he can survive humidity more than them, he is beating them, but not because he is the only person who can handle that heat and humidity. I've played in humidity, it's not that hard to survive it and it's just not an issue to the point where one side can win easily with stamina. Maybe if GSG was only playing people who flew down from Canada or Colorado etc. who had never ever been in humidity, maybe that would be a slight bump.
Serve and Volley, it's the easiest style to play if you are worried about heat and humidity. If, big if, if GSG's main weapon was just surviving humidity, the worst strategy would be to baseline battle him. But, in my opinion, his fitness is great and ability to get the back is obvious a strength, but that's not his ticket to winning, that style travels indoors and to other climates if it is good enough to work.
Heat and humidity, especially Atlanta, isn't a secret climate weapon lol, there are plenty of us that play in Texas, Arizona, Florida, Vegas, even So Cal.