* Old English is the language spoken in England from roughly the 5th to the 12th centuries. It is considered a distinct language from modern English and is very difficult for modern readers to understand.
* The Great Gatsby was written by F. Scott Fitzgerald in the 1920s and uses Modern English. While the language and style are characteristic of the period, it is easily understood by contemporary readers.
You may be thinking of other classic novels that utilize archaic language styles, like Shakespeare's plays, which are written in Early Modern English (Shakespearean English).