Relying only on dictionaries to understand how a word was understood in the recent past is the equivalent of placing one's hand on a sick child's forehead to gauge temperature instead of the electronic thermometer that is in your drug cabinet. (1)
The New York Times index is free and readily searchable. To understand quickly Fitzgerald's possible intentions of his 1925 employment of "holocaust" (referencing the deaths of Gatsby and Wilson) search "holocaust" in that index for some appropriate time period.
Searching June 1 1923 to June 1 1927 produces thirty "holocausts", most reference war or an aspect of war but the range of referents is rather wide e.g.:
"5,000 SONGBIRDS PERISH IN A FIRE ... The roosters kept crowing all through the bird holocaust." [March 16 1926]
"7 YANK RUNS IN 8TH BURY RED SOX ... It was a nice little holocaust, with the Yanks scoring seven runs in one inning." [May 31 1927]
The Library of Congress has a free - somewhat awkward - searchable index of hundreds of US newspapers with a cut off date of 1924. To access that index google search >Chronicling America<.
Random 'finds' from the hundreds of "holocausts" employed in American newspapers in 1922:
"... another word war would be the most appalling holocaust since the beginning of time." [May 26 1922, Seattle Star]
And "... the approach of the Christmas holidays has come to mean ... a commercial holocaust." [Dec 13 1922, Seattle Star]
Dictionaries are not to be trusted on word history or use.
The 1989 Oxford English Dictionary's entry "holocaust" does not mention the employment of "holocaust" in the early 1960's as THE referent to the then actively feared nuclear destruction.
("Stevenson ... asked whether such blunders might not 'carelessly, accidentally, trigger the holocaust.' The former Democratic Presidential candidate, in an address to the Conference on World Tension" --- New York Times Index, May 13 1960)
And: The Oxford Dictionary leaves the impression that "holocaust's" earliest employments referenced almost entirely sacrifices to the Judeo-Christian god and that such "authorized" holocausts swamped any referents of the word to pagan sacrifices --- both in the past and in modern usage.
Yet an educated Protestant English speaker in the period 1600-1940 would have encountered "H/holocaust" as a referent to a pagan sacrifice in Latin and Greek texts but never in a Protestant Bible. Two classical pagan holocausts: 1) "With Holocausts he Pluto's altar fills." (Dryden's 1697 translation of the Aeneid, Book VI) And 2) "Jupiter opposes you ... you have neglected to ... offer ... a holocaust." (Smith's 1827 translation of Xenophon's Anabasis, p. 540 -- Xenophon's "holocaust" is the earliest recorded employment of the word (c. 365 BCE); and Xenophon's Anabasis is probably the most read text by students of classical Greek.
A significant modern "holocaust" in its pagan sense: "For him [Eli Wiesel] ... God is dead ... the God of Abraham, of Isaac, of Jacob ... has vanished forevermore ... in the smoke of a human holocaust exacted by Race, the most voracious of all idols." (From Francois Mauriac's introduction to Wiesel's NIGHT (1960) -- Mauriac received the Nobel for literature in the 1950's, Wiesel picked up a Nobel for peace in the 1980's. Wikipedia reports 6 million copies of NIGHT had been sold in the USA by 2011.)
(The above few paragraphs of the footnote are largely drawn from the appendix of the web article on the word holocaust mentioned in earlier posting. The appendix critiques the Oxford Dictionary's "holocaust" entry in some detail. For the article and its appendix google >Petrie word holocaust<)