Addition Text to Post Titles (Custom Post Types) in RSS

Since you’re defining $post_type, why not just use it instead?

function wpbeginner_titlerss($content) {
    global $wp_query;
    $post_type = $wp_query->post_type;

    if( $post_type == 'essays' ) {
        $content="Essay: ".$content;
    }
    elseif ( $post_type == 'gallery' ){
        $content="Gallery: ".$content;
    }
    elseif ( $post_type == 'games' ){
        $content="Game: ".$content;
    }
    else {
        $content = $content;
    }
    return $content;
}
add_filter( 'the_title_rss', 'wpbeginner_titlerss' );

(This assumes you have created custom post types, essays, gallery, and games.)