In order to believe this you have to ignore VAST quantities of historical evidence. It’s no different than claiming Percy Shelley wrote "Frankenstein".
Now, it is true that Scott used Zelda’s diary as a source of some parts of "Gatsby", but Zelda did not write the book.
F. Scott Fitzgerald didn't write The Great Gatsby. His wife Zelda Fitzgerald did. He just had the right gender at the time to publish it. Read Zelda's own novel, Save Me the Waltz, and tell me I'm wrong. I have never been more certain of anything in my life.