<?php
add_filter("gform_pre_render", "ictr_populate_boeken");
add_filter("gform_admin_pre_render", "ictr_populate_boeken");
function ictr_populate_boeken($form, $form_id = 14, $field_id = 16, $field_type = 'checkbox') {
    global $post;

    if(!in_array($form['id'], array(14, 15))) {
        return $form;
    }

    $args_posts = array(
        'post_type' => 'boek',
        'orderby' =>'titel',
        'order' => 'ASC',
        'numberposts' => -1
    );
    $posts = get_posts($args_posts);

    // Creating drop down item array.
    $items = array();

    //Adding post titles to the items array
    foreach($posts as $i => $item) {
        $tmp = array(
            'value' => $item->post_title,
            'text' => $item->post_title
        );
        if($item->ID == $post->ID)
            $tmp['isSelected'] = true;

        $items[] = $tmp;
    }

    foreach($form['fields'] as &$field) {
        if($field['id'] == $field_id) {
            $field['choices'] = $items;
            $field['type'] = $field_type;
        }
    }

    return $form;
}

?>