functions.php

<?php
/**
 * Grid Loop on Portfolio archive
 *
 * @author Bill Erickson
 * @link https://github.com/billerickson/Genesis-Grid/wiki/Home
 *
 * @param bool $grid, whether to use grid loop
 * @param object $query, the WP Query
 * @return bool
 */
function be_grid_loop_on_portfolio( $grid, $query ) {
  if( is_tax() && $query->is_main_query() )
		$grid = true;

	return $grid;
}

add_filter( 'genesis_grid_loop_section', 'be_grid_loop_on_portfolio', 10, 2 );