When you live in the city, kids walk around by themselves more often. And Gus looks more about seven.
If Trolls hate flowers and all things that grow, why would they save the most barren land imaginable (New York) as punishment instead of living there themselves?
There is a possible explanation for this; it seems only Llort was aware of New York (although how he, of all people, would know about it is another story), and that the other trolls are unaware of or are apathetic to knowledge of the human world, let alone know about any cities from it. And prior to Llort proposing to Gnorga the idea of banishing Stanley, they were with Gnorga on turning Stanley into stone as punishment, and had no alternatives otherwise.
How did Gnorga even know where to banish Stanley to when Llort didn't even tell her where it was to begin with and only gave a general description of how horrible it was? And how did Llort even learn about New York in the first place, since the Troll Kingdom is clearly a far ways off from any place humans would be around? Is it possible he somehow picked up a piece of gossip of the outside world (since the idea of other trolls knowing about the human world isn't impossible, especially since Stanley mentions offhand that his mother used to read him stories about humans)?
While Gnorga going to New York without bringing other trolls can be written off due to her impatient, temperamental personality, did she even consider that sending a Tornado to destroy Central Park, located in one of the most densely populated cities on Earth, all just to capture a rogue troll and two kids, would attract a lot of unwanted attention (even if no one did appear to notice the chaos caused, even after Gnorga is defeated, for reasons not explained)?