When mentioning a book title like "The Great Gatsby" in a sentence, the title should be enclosed in quotation marks if you are writing in a format that requires such punctuation (like in American English). For example:
- I just finished reading "The Great Gatsby" by F. Scott Fitzgerald.
In contrast, if you are using italics, the title can be italicized instead of being placed in quotation marks:
- I just finished reading The Great Gatsby by F. Scott Fitzgerald.
Both formats are correct; the choice between them depends on your writing style and the conventions you are following.