Taken from: desgrange.net

Remember to

sudo a2enmod rewrite
sudo a2enmod headers
sudo apachectl configtest

in your <VirtualHost>:

<Directory */path/to/my/blog/*>
    RewriteEngine on
    # Gzip
    # If the web browser accept gzip encoding… 
    RewriteCond %{HTTP:Accept-Encoding} gzip
    # …and the web browser is fetching a probably pre-compressed file…
    RewriteCond %{REQUEST_URI} .*\.(css|html|js)
    # …and a matching pre-compressed file exists… 
    RewriteCond %{REQUEST_FILENAME}.gz -s
    # …then rewrite the request to deliver the gzip file
    RewriteRule ^(.+) $1.gz
    # For each file format set the correct mime type (otherwise gzip mime type is returned) and prevent Apache for recompressing the files
    RewriteRule "\.css\.gz$" "-" [T=text/css,E=no-brotli,E=no-gzip]
    RewriteRule "\.html\.gz$" "-" [T=text/html,E=no-brotli,E=no-gzip]
    RewriteRule "\.js\.gz$" "-" [T=application/javascript,E=no-brotli,E=no-gzip]
    <FilesMatch "\.(css|html|js)\.gz$">
        # Serve correct encoding type
        Header set Content-Encoding gzip
        # Force proxies to cache gzip & non-gzip files separately
        Header append Vary Accept-Encoding

Whilst scouring the web for useful pelican plugins, I came across gzip_cache

The idea is that you have a precompressed html.gz / css.gz / js.gz files alongside your html / css / js files.

The above apache2 config should get Apache to serve these already compressed files instead of compressing on the fly.