add_filter('gform_pre_render_6', 'populate_coach');
function populate_coach($form){
    foreach($form['fields'] as &$field){
        if($field['type'] != 'select' || strpos($field['cssClass'], 'populate-agenda-coach') === false)
            continue;
        // you can add additional parameters here to alter the posts that are retreieved
        // more info: http://codex.wordpress.org/Template_Tags/get_posts
        $posts = get_posts('category=221&numberposts=18');
        // update 'Select a Post' to whatever you'd like the instructive option to be
        $choices = array(array('text' => 'Choisir une date', 'value' => ' '));
        foreach($posts as $post){
            $choices[] = array('text' => get_post_meta($post_id, 'custom_field_name', false) , 'value' => get_post_meta($post_id, 'custom_field_name', false) );
        }
        $field['choices'] = $choices;
    }
    return $form;
}