add_filter( 'gform_pre_render_2', 'populate_posts' );
add_filter( 'gform_pre_validation_2', 'populate_posts' );
add_filter( 'gform_pre_submission_filter_2', 'populate_posts' );
add_filter( 'gform_admin_pre_render_2', 'populate_posts' );


function populate_posts( $form ){

  foreach ( $form['fields'] as &$field ) {
    if ( $field->type != 'select' || strpos( $field->cssClass, 'populate-posts' ) === false ) {
      continue;
    }

    $posts = get_posts(array(
      'numberposts' => -1,
      'post_type'     => 'staff',
    ));

    $choices = array();

    foreach ( $posts as $post ) {
      $choices[] = array(
        'text'  => $post->post_title,
        'value' => $post->ID, //** <- I am using post ID , because url parameter need to be the post id. also i need to send post title when submit the form
      );
    }

    $field->placeholder = 'Select Staff Member';
    $field->choices = $choices;
  }

  return $form;

}