Hello, @Mintyy, and welcome to the SAS!
You’ve gotten excellent advice from our friends Hobbes & Pep - no surprise, since they’re among the most-seasoned & savvy vets among us - so it sounds like you’re kitted up for moving forward.
Still, I did want to mention that Amazon actually does support a mechanism for re-directing improperly-assigned GTINs; internally, the Amazon terminology (aka ‘Amazonese’) for this procedure is “cleaving.”
As Hobbes alludes, it doesn’t always work, and/or could well prove not worth the bother in many a circumstance, but it can be a useful functionality if one can demonstrate ownership of the improperly-assigned GTIN.
A few members of the SAS created several helpful ‘ASIN-cleaving tutorials’ on this subject back in the days before the advent of the NSFE, and some of them are still available - such as the ones linked in our friend @Haegan2005’s 072024 (55th Moon-landing Day) SAS thread here:
https://test.sellersasksellers.com/t/while-i-know-we-can-cleave-an-asin-i-can-not-find-how/4389