add_filter("gform_pre_render", "populate_dropdown");
function populate_dropdown($form){
	if($form["id"] != FORM_EDIT_POST) { return $form; }

	if( isset( $_GET['postid'] ) ) {
		$postid = esc_attr( $_GET['postid'] );
		$post = get_post($postid);

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

			if( $field["type"] == 'post_category' ){

				$categories = get_the_category($postid);
				if($categories){
					foreach($categories as $category) {
						$selected_cat = $category->term_id;
					}
				}

				if(isset($selected_cat)) {
					$args = array(
						'orderby' => 'name',
						'order' => 'ASC'
					);
					$categories = get_categories( $args );
					foreach( $categories as $category ) {
						$is_selected = $category->term_id == $selected_cat ? true : false;
						$choices[] = array( "text" => $category->name, "value" => $category->term_id, "isSelected"=> $is_selected );

					}
					$field["choices"] = $choices;
				}
			}
		}
	}
	return $form;
}