Definitely. Ellen Hopkins is known for writing fiction. Her books typically involve fictional scenarios and characters that engage readers with compelling narratives.
I don't know for sure. The ending of such a manga can be quite unpredictable and might not have been revealed yet. You could try discussing it with other fans to get their speculations.
It's not based on a true story. The plot and characters of 'The Great Gilly Hopkins' were crafted by the writer for literary purposes rather than being drawn from real events or people.
I'm not really sure if they end up together. Different people might have different interpretations. Maybe you can dig deeper into the manga and form your own conclusion.