All pages in website download .GZ file

This happens for me, too, on the latest 4.8.1.

It appears that the headers are wrong. The webserver sends:

Content-Type: application/x-gzip

It should be sending:

Content-Type: text/html
Content-Encoding: gzip

Please let us know how to fix this—it makes for a dramatically worse user experience.

Some further experimentation suggests that this happens when apache2 on the server is configured to gzip at page load time (as it is in the Ubuntu mod_deflate config file). This results in double-compression. Disabling WP Super Cache compression fixes the issue, but does do the compression work on every page load.

So, this was always an issue for you? As @fishbreath said, disable compression in Apache. That should help. The plugin compresses files to save CPU on every request but if your site isn’t that busy you won’t notice a difference between the plugin doing it and Apache doing it.