Hi,
Thanks for using our plugin and reaching out to us.
WooCommerce doesn’t come with a brand field/attribute out of the box so you would indeed have to create such a brand attribute yourselves. This WooCommerce tutorial shows how to create such an attribute.
Once you created that new brand attribute you can use it on your product edit pages so you can give brand values to your products. Once you are done filling in the brands for your products you can map your new brand attribute to Google’s g:brand field in the field mapping section of our plugin.
Now, alternatively, when all your products have the same brand you can go for a much easier and faster way. You can add a so-called “static value” for your brand in the field mapping section of our plugin. This tutorial describes how to do so: https://adtribes.io/how-to-use-static-values-and-create-fake-content-for-your-product-feed/
The woosea brand (custom attribute) field you noticed in the drop-downs is a left-over from our previous paid version of the plugin. In-case you want to know more about that please contact our support team directly. Unfortunately, per the WordPress forum guidelines, we’re not allowed to discuss our paid plugins here.
Hope this helps?