Not sure where to post this, but I had the same question about how to change the default name of GTIN to UPC and I found it by clicking on WooCommerce in the left navigation of my site, then clicking Settings, then clicking Products, then there is a hard to see link called Inventory that you need to click. Near the bottom of the Inventory page is the GTIN Settings where you can change the GTIN label to UPC or EAN or whatever you need.
This plugin works great, but like most other WordPress/WooCommerce plugins it takes a little detective work to figure out how to make the plugins work as well as how to edit the plugins.
The developers put readme files in the ZIP files but I never extract the ZIP files I just add them to WordPress and hope there is a help/readme file installed into my WordPress.
