Why doesn't Gatsby inherit money from Dan Cody's will?

In chapter 6 of The Great Gatsby by F. Scott Fitzgerald, Nick tells the reader Gatsby's origin story: that his real name is James Gatz, and he grew up very poor. One summer he meets Dan Cody, a fifty-year-old millionaire. Dan Cody becomes a mentor to Gatsby and hires him, and the two sail together for five years. Nick tells the reader:

It might have lasted indefinitely except for the fact that Ella Kaye came on board one night in Boston and a week later Dan Cody inhospitably died.

Gatsby worked for Cody in many capacities, as a "steward, mate, skipper, secretary, and even jailor" who cared for Cody when he was drunk. The two were friends, and Cody left money to Gatsby. However,

He didn't get it. He never understood the legal device that was used against him, but what remained of the millions went intact to Ella Kaye.

Ella Kaye, Dan Cody's mistress, inherited the fortune instead of Gatsby. Why, exactly? Well, Fitzgerald leaves us with a few clues. The first has to do with Ella Kaye's character. In the detail that Cody dies a week after she boards the yacht, it is implied that she might have something to do with his death. Fitzgerald also tells us that women like Ella Kaye pursued Cody simply for his money:

The transactions in Montana copper that made him many times a millionaire found him physically robust but on the verge of soft-mindedness, and, suspecting this, an infinite number of women tried to separate him from his money. The none too savory ramifications by which Ella Kaye, the newspaper woman, played Madame de Maintenon to his weakness and sent him to sea in a yacht, were common knowledge to the turgid sub-journalism of 1902.

Ella Kaye is a shady character who has a plan in place to take Cody's money, and so Gatsby is unable to claim it.

We can also infer that Gatsby was unable to fight for the money due to his status in life. Gatsby didn't understand "the legal device" that led to Ella Kaye receiving the money, and we know that Gatsby would not have been able to afford a lawyer to fight for his right to the money.

More than anyone else, the wealthy copper magnate Dan Cody is Gatsby's mentor. It was he who first provided him with a tantalizing glimpse into the lifestyles of the rich and famous, who showed him how to achieve success and live the American Dream. He took Jay on as his personal assistant and over the next five years, shared his extensive business expertise and wisdom with the young man. If anyone's responsible for turning plain old James Gatz into Jay Gatsby it's Dan Cody.

Dan Cody forges a close relationship with Jay, so much so that he leaves him $25,000 in his will. Unfortunately, Jay never sees a cent of the money as Dan's greedy mistress Ella Kaye prevents him from getting what's rightfully his.

In The Great Gatsby, why doesn't Gatsby receive Dan Cody's inheritance?

In the novel The Great Gatsby, by F. Scott Fitzgerald, we read about the American dream in the 1920's.  The main character was Jay Gatsby and the main topic of the narration by Nick Carraway.  It is through Nick that the reader learns of Jay's involvement with the eccentric, Dan Cody.  Dan liked Gatsby's energy and eagerness to learn.  He trained Jay and took him places that Gatsby would never have gone. 

"Gatsby spots the moored yacht of Dan Cody. In an action that changes the young boy's life, Cody welcomes him aboard his yacht and introduces him to fine living. Gatsby becomes the protege of the wealthy gold miner and lives with him until Cody dies."

Gatsby doesn't receive the Cody fortune because his Cody's family swoops in and takes the money. This story is recounted in chapter 8 of the novel.  Nick is home from the city and has seen the accident scene.  He goes to Gatsby to encourage him to leave town but instead Gatsby insists on protecting Daisy and it is at this point in the novel that Gatsby shares the story of Dan Cody.

In The Great Gatsby, how does Gatsby lose his inheritance from Dan Cody?

We first meet Dan Cody in Chapter Five when Nick asks Gatsby about a photograph showing an elderly man in a yachting outfit. Gatsby states that the old man is now deceased but was his best friend years ago. Later, we learn that Gatsby once lost a claim to an inheritance of twenty-five thousand dollars from Cody because of Cody's mistress.

In Chapter Six we learn more about Dan Cody and Gatsby's history with the older man. Gatsby first met Dan Cody when the latter's yacht dropped 'anchor over the most insidious flat on Lake Superior.' On a whim, Gatsby had rowed out to warn Cody that his yacht was anchored in a precarious location, thus earning him the older man's gratitude and friendship. At the time Gatsby met Cody, he had just left a short, two week stint at the small Lutheran college of St. Olaf in southern Minnesota. Perturbed by the lowly janitorial work he had had to perform in order to pay his way through school, Gatsby had quit college in rebellion. Listless, he had then found work as a 'clam digger' and a 'salmon fisher' along the south shores of Lake Superior.

Dan Cody is described as a successful man who made his millions through metal prospecting. Because of Gatsby's agreeable personality, Cody took a liking to the younger man; working in a 'vague personal capacity,' Gatsby found himself a 'steward, mate, skipper, secretary, and even jailor' to his older benefactor. Dan Cody placed great trust in Gatsby to protect and to care for him when he was under the influence of alcohol. The affable arrangement went on for five years until Ella Kaye, a reporter, came into Dan Cody's life.

Besides an inordinate obsession with alcohol, Dan Cody was also fascinated with beautiful women. According to gossip among influential circles, Ella Kaye may have played a part in Dan Cody's premature death.

The none too savory ramifications by which Ella Kaye, the newspaper woman, played Madame de Maintenon to his weakness and sent him to sea in a yacht, were common knowledge to the turgid journalism of 1902.

Accordingly, Gatsby never received his twenty five thousand dollar inheritance from Cody; what 'remained of the millions went intact to Ella Kaye.' The wily mistress had managed to use legal maneuvers to defeat Gatsby's claim. The allusion to Madame de Maintenon is significant, as the historic figure had risen from humble beginnings to the position of royal wife through clever positioning and Machiavellian intrigue. Likewise, Ella Kaye, from humble beginnings, had managed to win millions of dollars in inherited money simply by using her relationship with a debauched, older lover as leverage. Despite Ella Kaye's actions, Gatsby managed to reinvent himself into a man of wealth and glamor.