Scott Bolinger

GTINs (Global Trade Item Numbers) including UPC, EAN, and ISBN are globally recognized unique identifiers for products.
They are required to list your product on Google and Amazon, this plugin allows you to add your GTIN and display it on your product page.
Supports:
<ul>
<li>Simple products</li>
<li>Variable products</li>
<li>Composite products</li>
<li>Product bundles</li>
</ul>
Features:
<ul>
<li>Add GTIN for main product</li>
<li>Add unique GTINs for product variations</li>
<li>Option to display GTIN on the single product page</li>
<li>Change GTIN to any other text</li>
<li>Add GTIN to structured product data</li>
<li>Works with product feeds like Google Shopping and more (requires a feed plugin)</li>
</ul>

WooCommerce UPC, EAN, and ISBN

Awesome plugin with great code

Add GTIN codes like UPC, EAN, and ISBN to your WooCommerce products.

João Carmo Pereira

This plugin adds a numeric GTIN field to Simple Products and Product Variations if they exist. This field can be used on product pages and for product feeds through shortcode. 
Additionally, the plugin provides an options menu where users can delete all GTIN entries from the database before uninstalling the plugin.

Carmo Product GTIN for WooCommerce

This plugin will add a numeric GTIN field to Simple Products and Product Variation if they exist.  This field can be used via shortcode [carmogtin] on …

WooCommerce UPC, EAN, and ISBN vs Carmo Product GTIN for WooCommerce