In 1928, Gruger was assigned by the Saturday Evening Post to illustrate a long and tedious detective story. Rather than draw another dozen pictures of English gentlemen sitting around tables in a parlor, Gruger concoted a wraith-like apparition (not a character in the original story) to embody hidden mysteries.
Personally, I think Gruger just felt like drawing a cool figure in flowing robes. Look at how much fun he had with these pictures:
Another illustration from the same story:
I don't have all of the originals from the story to scan (The two above came from our friends at Taraba Illustration Art ) but if you look at the following printed versions from the Post, you can get a sense for how Gruger drew each wraith distinctively, each with its separate dramatic flourish:
These are not your run-of-the-mill Halloween ghosts. Here you are seeing Gruger's vivid imagination in action.