Simple, don't submit the bar code. Only bar codes from the manufacturer can be submitted. Self generated bar codes are not allowed.
If you can't set it up via your app, you can use feed rules, and set GTIN to clear. More info: https://support.google.com/merchants/answer/7450276?hl=en
To learn more about bar codes : https://support.google.com/merchants/answer/6324461
You need cancel the data source and click on add modification and set it to clear.
Also remove brand and mpn values
And set identifier exists to no
Then process the feed.
Hope this helps.