function gist_function( $atts ) {
        $a = shortcode_atts( array(
                'url' => 'A_DEFAULT_URL'
        ), $atts );

        return '<script src="' . esc_attr($a['url']) . '.js"></script>';
}
add_shortcode( 'gist', 'gist_function' );

[gist url="your gist url"]