Adding Gravatar to rss feed

The point is, that the format have no tags for a image to a user. You must use a other tag to inlcude the avatar of a author, not the author tag. The author tag is only for a string, not markkup or images. But you can add content ot the desciption and this is easy on the filter the_content_feed.

A small example, there add content to this part in the feed.

add_filter( 'the_content_feed', 'fb_plugin_function_name' );
/**
* @param  $content Content of post
* @return string
*/
function fb_plugin_function_name( $content ) {

   $content .= 'Total ' . str_word_count( $content ) . ' words';
   return $content;
}