Sentence:  He was handing her something in an envelope, and she was saying “Oh, Jeeves, you've saved a human life,” and he was saying “Not at all, miss.” The <WSD>gist</WSD>, of course, escaped me, but I had no leisure to probe into <WSD>gists</WSD>. 
Meaning of the word marked with <WSD> tags:  The most essential part; the main idea or substance (of a longer or more complicated matter); the crux of a matter; the pith.