wp_gist

wp_embed_register_handler( 'github-gist', '#https?://gist\.github\.com/([0-9]+)#', 'gist_embed_handler' );
add_shortcode( 'gist', 'gist_shortcode' );
function gist_embed_handler( $matches, $attr, $url, $rawattr ) {
  return gist_shortcode( $attr, $url );
}
function gist_shortcode( $atts, $content = '' ) {
  if ( empty( $atts[0] ) && empty( $content ) )
    return '<!-- Missing Gist ID -->';
  $id = ( ! empty( $content ) ) ? $content : $atts[0];
  if ( ! is_numeric( $id ) )
    $id = preg_replace( '#https?://gist.github.com/([0-9]+)#', '$1', $id );
  $id = (int) $id;
  if ( ! $id )
    return '<!-- Invalid Gist -->';
  $embed_url = "https://gist.github.com/{$id}.js";
  if ( ! empty( $atts['file'] ) )
    $embed_url = add_query_arg( 'file', urlencode( $atts['file'] ), $embed_url );
  return '<script src="' . esc_url( $embed_url ) . '"></script>';
}