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 …

JS Morisset

Provides additional input fields in the WooCommerce “Product data” metabox and under the product webpage “Additional information” section:
<ul>
<li>MPN (Manufacturer Part Number)</li>
<li>ISBN</li>
<li>GTIN-14</li>
<li>GTIN-13 (EAN)</li>
<li>GTIN-12 (UPC)</li>
<li>GTIN-8</li>
<li>Net Length / Depth</li>
<li>Net Width</li>
<li>Net Height</li>
<li>Net Weight</li>
<li>Fluid Volume</li>
</ul>
Fully compatible with the WooCommerce “GTIN, UPC, EAN, or ISBN” input field – extends the basic WooCommerce global unique ID feature with several additional, and more precise, global unique ID input fields.

The SSO > WC Metadata settings page allows you to enable or disable each product metadata (aka custom field), along with customizing the label and placeholder values for the available languages (aka WordPress locales).
The product global identifier values (ie. MPN, ISBN, GTIN-14, GTIN-13, GTIN-12, and GTIN-8) are searchable from the front-end webpage and the admin WooCommerce Products page.
The WPSSO Schema Product Metadata for WooCommerce add-(aka WPSSO WCMD) on provides Schema (aka Schema.org) mpn, gtin14, gtin13, gtin12, gtin8, gtin, productID isbn, depth (aka length), width, height, weight, and additionalProperty fluid_volume values for Google Rich Results, Rich Snippets, and Structured Data.
<h3>Includes WooCommerce Fluid Volume Units</h3>
Includes a Fluid volume unit option in the WooCommerce > Settings > Products settings page:
<ul>
<li>ml</li>
<li>cl</li>
<li>l</li>
<li>US tsp</li>
<li>US tbsp</li>
<li>US fl oz</li>
<li>US cup</li>
<li>US pt</li>
<li>US qt</li>
<li>US gal</li>
</ul>
<h3>Additonal Meta Tags and Schema Markup</h3>
Automatically provides additional Open Graph product meta tags for enabled product metadata:
<ul>
<li>product:ean</li>
<li>product:isbn</li>
<li>product:mfr_part_no</li>
<li>product:upc</li>
<li>product:weight:value</li>
<li>product:weight:units</li>
</ul>
Automatically provides additional Schema Product and Offer properties for enabled product metadata:
<ul>
<li>mpn</li>
<li>gtin14</li>
<li>gtin13</li>
<li>gtin12</li>
<li>gtin8</li>
<li>productID isbn</li>
<li>depth (aka length)</li>
<li>width</li>
<li>height</li>
<li>weight</li>
<li>additionalProperty fluid_volume</li>
</ul>
<h3>WPSSO Core Required</h3>
WPSSO Schema Product Metadata for WooCommerce is an add-on for <a href="https://wordpress.org/plugins/woocommerce/" rel="ugc">WooCommerce</a> and the <a href="https://wordpress.org/plugins/wpsso/" rel="ugc">WPSSO Core plugin</a>, which creates extensive and complete structured data to present your content at its best for social sites and search results – no matter how URLs are shared, reshared, messaged, posted, embedded, or crawled.

WPSSO Schema Product Metadata for WooCommerce

MPN, ISBN, GTIN-8, UPC, EAN, GTIN-14, net dimensions, and fluid volume for better WooCommerce Schema markup.

Carmo Product GTIN for WooCommerce vs WPSSO Schema Product Metadata for WooCommerce