To clarify, every page load (.html file) was downloaded as a .gz file instead of being served as an HTML file and displayed as a web page in the browser.

Having once made a similar mistake myself, I assume what they mean is that it was downloading everything as a .gz—that is, the browser was asking users “where would you like to save index.html.gz?” instead of showing the homepage. (This happens when you precompress a static site for performance, but forget to tell the server that gzip should be negotiated as a Content-Encoding instead of a Content-Type.)