Value can be loaded to the GTIN field in the article archive (ART) using the ARTUPLOAD job (INP/14542)

using the 'EAN' - field. 

The GTIN - Global Tade Item Number is a 14 digit identifier for trade items (comprising among others of the former EAN International and Uniform Code Council).

Examples of types of GTIN are: UPC, EAN, JAN, ISBN

This information can be found if you hover over the information icon on the GTIN field:

Input documentation for INP/14542 ARTUPLOAD: