The best way to go about this is to research plagiarism laws. According to LegalZoom, plagiarism would entail:

"If you use another person's work and do not attribute that work to the author, including copying text verbatim, paraphrasing a phrase or summarizing an idea, you are essentially committing plagiarism. Plagiarism usually occurs when a writer fails to:
-cite quotes or ideas written by another author;
-enclose direct text in quotes; or
-put summaries and/or paraphrases in his or her own words."

According to Ithenticate, "Copyright laws are absolute. One cannot use another person’s material without citation and reference" and "In the case where an author sues a plagiarist, the author may be granted monetary restitution."

I would absolutely attribute the story to Gladwell and perhaps even cite the speech in your bibliography, as well, just to be safe.