Often used humorously, the informal phrase gory details denotes the explicit or most intimate details of something.

This phrase originally referred to accounts or representations of acts of violence and bloodshed.

These are, in chronological order, the earliest occurrences of the phrase gory details that I have found:

1-: From Our New York Correspondence, by ‘Knickerbocker’, published in the Sunday Dispatch (Philadelphia, Pennsylvania, USA) of Sunday 13^th March 1859:

We had a shocking murder, the other day, in this quarter. Such calamities are common enough with us, I trow—so commonplace that I do not often trouble you with a note of their gory details unless they exhibit some feature of novelty or extravagance.

2-: From a letter by ‘Sigma’, published in the Boston Evening Transcript (Boston, Massachusetts, USA) of Monday 4^th June 1860—in this letter, the adjective brutal precedes gory:

It would be absurd to deny the existence of that morbid curiosity, which leads a very large part of the community to read, with no little gusto, the minute details of bullfights, dogfights, manfights, domestic broils, and divorces for the cause of adultery; and, when a clergyman happens to be the guilty party, the interest is marvellously increased.
It is clearly a very grave question to what extent the conductor of a respectable public journal may furnish this species of rawhead and bloody bones literature to his customers. […]
[…]
These conflicts, between human beings, are utterly degrading; and the sensations, produced among the beholders, and, in no small degree, among the readers of these detailed accounts, clearly show, that men, said to be created in the image of their Maker, and bull dogs are not so entirely dissimilar, after all. Perhaps the late Fisher Ames ¹ was correct, in his opinion, that everybody has a little devil in him, about as big as a woodchuck. If it be so, it is clear, that the process of casting him out cannot consist in the patronage of prize fights, either by bestowing money upon the combatants, or, by publishing the brutal, gory details of their battles. It is humbling to believe, that these disturbing publications have no worthier origin, than the desire to secure the popularity of the journals, in which they appear.

¹ Fisher Ames (1758-1808) was a U.S. essayist and politician.

3-: From the review of A Bundle of Ballads (London: Tinsley, Brothers, 1864), by the British author George Alfred Lawrence (1827-1876)—review published in The Athenæum: Journal of English and Foreign Literature, Science, and the Fine Arts (London, England) of Saturday 23^rd January 1864:

‘Unknown, yet well-known,’ is an attempt to immortalize the war-maddened rifleman, whose determination to win his way into Badajos, 1812 ², led him to grovel beneath the chevaux-de-frise ³ planted in the breach, and allow himself to be battered to pieces. We differ from the author in thinking that this is the right kind of heroism to be immortalized in verse, and dissent still more from him with regard to the introduction of gory details into the episode that is so briefly related by Napier. If we look to old models, we find the best of them avoid such details.

² The Siege of Badajoz was one of the bloodiest engagements of the Napoleonic Wars. An Anglo-Portuguese army besieged Badajoz, in Spain, from 16^th March to 6^th April 1812, and forced the surrender of the French garrison.
³ The French noun cheval de frise (plural chevaux de frise) designates a defensive appliance, consisting of a large joist traversed with projecting spikes, used to obstruct the passage of cavalry. This French noun means, literally, horse of Friesland, because the appliance was first employed by the Frisians in their struggles for freedom during the latter half of the 17^th century to supply their want of cavalry.

4-: From a theatrical review published in The Daily Dramatic Chronicle (San Francisco, California, USA) of Monday 22^nd June 1868—in this review, the adjective ghastly precedes gory:

Oliver Twist only drew a fair crowd on Saturday night. As “Nancy Sykes” Lucille Western achieved her first successful personation; we have never witnessed anything so terrible in the drama but in one instance—when we witnessed Julia Dean Hayne’s personation of the same disgusting role. With the same faults of style and mannerisms such as reading in short sentences, jerked out, a constant winking of the eyes, and frequently slapping her thighs with her hands, which we have noticed before, Miss Western invested the character with a terrible interest, and infused into it a vein of pathos that was so admirably depicted as to win the entire sympathies of her auditors and to absorb their riveted attention whenever she was on the stage. Her last scene, where she is killed by her lover “Sykes,” was the very acme of blood-curdling horrors, and in its ghastly and gory details was sickening to behold. It was a terribly real piece of acting, and well done; but who that witnessed it would want to do so again. In England such demoralizing exhibitions are prohibited by law, they should be here.

5-: From England and the War, published in The Hertfordshire Express, and General Advertiser for the Counties of Beds., Bucks., Cambs., Essex, Hunts., and Middlesex (Hitchin, Hertfordshire, England) of Saturday 30^th July 1870:

It seems as though a horrible appetite for blood had taken hold on Europe. People are impatient for news of carnage; they seem to forget that with all the modern progress in the art of wholesale murder, battles cannot be fought by telegraph, and the opposing forces, though eager for the fray, cannot begin butchering each other until they can be brought together. Even then the vampire appetite will only be whetted and aggravated by brief telegrams telling how many thousands have been killed and wounded; and the “thirst for news” cannot be slaked with the gory details until some days after the fighting, when the special correspondents have had time to serve up the dainty fare, garnished with their most truculent forms of speech.

6-: From The Memphis Avalanche (Memphis, Tennessee, USA) of Monday 30^th June 1873:

—Sunday is not usually a magnificent day for news, either from the police courts or other sources, but Monday—ah! then does the flightsome Faber leap over the white paper, chronicling the slashed gullet or the suicide’s last end, and drips in a perfect stream the gory details of the last beer-garden’s murder, or the jealous stabbing affray in Pinch, Hell’s Half-Acre, or the Wolf river flats. Yesterday was Sunday.

7-: From The Literary World: A Review of Current Literature (Boston, Massachusetts, USA) of November 1875:

Murders and grave crimes, in real life, at least, are quite without our province. But we feel impelled to inquire if the recent frightful frequency of murder in New England, often attended by circumstances of almost unequalled atrocity,—if this seeming epidemic of blood-shedding is not partly due to the unwise newspaper policy which makes capital of these horrors? To the journalist of this enlightened century, the news of a murder is a boon; to him, the red drops of the victim’s blood represent dollars, or, at least, dimes; and the more dreadful the murder, the more lucrative the dissemination of its particulars. Such reading pleases a very large class, and that class is mainly composed of men and women who are under no moral restraint, or very little. To many of them these murders take on an heroic aspect; the murderer is a gallant and admirable spirit. To others, the gory details bring a spell, a fascination, which obscures their moral sense, and suggests the idea of imitation. These propositions may seem extravagant, but they are not; any one who knows how weak is average uncultured human nature, how impressible by dramatic influences, must admit that to a vast number of human beings, the news of a great crime is fraught with dangerous possibilities. Who will inaugurate the reform by closing the columns of his newspaper to this perilous stuff? Let the clergy preach against this dreadful literature; let humane editors write against it; let good citizens discourage it by tabooing the papers which disseminate it.