Found this thread as I was searching for the same thing, just adding this as a code snippet from the link provided by David did the trick for me – no GP specific CSS needed.
/**
* @snippet Change No. of Thumbnails per Row @ Product Gallery | WooCommerce
* @how-to Get CustomizeWoo.com FREE
* @sourcecode https://businessbloomer.com/?p=67117
* @author Rodolfo Melogli
* @testedwith WooCommerce 3.0.5
*/
add_filter( 'woocommerce_single_product_image_gallery_classes', 'bbloomer_5_columns_product_gallery' );
function bbloomer_5_columns_product_gallery( $wrapper_classes ) {
$columns = 5; // change this to 2, 3, 5, etc. Default is 4.
$wrapper_classes[2] = 'woocommerce-product-gallery--columns-' . absint( $columns );
return $wrapper_classes;
}
Thanks
Greg