Populate drop down with categories

// http://www.gravityhelp.com/forums/topic/populate-a-drop-with-categories-from-current-post
// update the '33' to the ID of your form
add_filter('gform_pre_render_33', 'populate_cats');
function populate_cats($form){
        // loop through each of the form fields...
        foreach($form['fields'] as &$field){

                // ... and skip this function if this is not a Select box or a cssClass of populate-categories is not present
                // be sure to add the CSS Class to your select box Advanced tab
                if($field['type'] != 'select' || strpos($field['cssClass'], 'populate-categories') === false)
                        continue;

                // here's where we populate the drop down
                // update 'Rim Size' to whatever you'd like the instructive option to be
                $choices = array(array('text' => 'Rim Size', 'value' => ' '));

                // the WordPress function get_the_category will return all categories for the current post
                // http://codex.wordpress.org/Function_Reference/get_the_category
                foreach((get_the_category()) as $category) {
                        $choices[] = array('text' => $category->cat_name, 'value' => $category->cat_name);
                }
        // populate $field['choices'] with our array of choices
        $field['choices'] = $choices;
        }
        // always return the $form
        return $form;
}