How did Ginger die? All I know is that she did die at some point in the book, but how did she die?

From the text, we can infer that Ginger died from overwork and abuse.

Just then her driver came up, and with a tug at her mouth backed her out of the line and drove off, leaving me very sad indeed.

A short time after this a cart with a dead horse in it passed our cab-stand. The head hung out of the cart-tail, the lifeless tongue was slowly dropping with blood; and the sunken eyes! but I can't speak of them, the sight was too dreadful. It was a chestnut horse with a long, thin neck. I saw a white streak down the forehead. I believe it was Ginger; I hoped it was, for then her troubles would be over. Oh! if men were more merciful they would shoot us before we came to such misery.