// replace 1 below with your form id and populate_services with your parameter for dynamic population

add_filter( 'gform_pre_render_1', 'populate_services' );
function populate_services( $form ) {

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

        if ( $field->type != 'select' || strpos( $field->cssClass, 'populate-services' ) === false ) {
            continue;
        }
        $choices = array();
        $terms = get_terms( 'TAXONOMY_HERE', array('hide_empty' => 0));
        foreach( $terms as $term ) {
            $choices[] = array( 'text' => $term->name, 'value' => $term->name );
        }
        $field->placeholder = 'Select a service';
        $field->choices = $choices;

    }

    return $form;
}