Dynamically Populating Drop Down Fields

// update the '51' to the ID of your form.
add_filter('gform_pre_render_6', 'populate_posts');
function populate_posts($form){

    foreach($form['fields'] as &$field){

        if($field['type'] != 'select' || strpos($field['cssClass'], 'populate-posts') === 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('numberposts=-1&category=1&orderby=title&order=ASC&post_status=publish');

        // update 'Select a Post' to whatever you'd like the instructive option to be
        $choices = array(array('text' => 'Select a Post', 'value' => ' '));

        foreach($posts as $post){
            $choices[] = array('text' => $post->post_title, 'value' => $post->post_title);
        }

        $field['choices'] = $choices;

    }

    return $form;
}