IMHO - adding a few lines to a template override, is far easier and more manageable than using customfields
e.g. for the category view:-
\templates\yourtemplate\html\com_virtuemart\sublayouts\products.php
//GTIN may be empty for a product
if(!empty($product->product_gtin){
echo $product->product_gtin;
}
echo $product->product_sku;
You can then forget about customfields and the data will be shown if entered on the product record