Hey JustinCoziness,
Ideally the products with different sizes should have different GTIN values.
But since you don’t have GTINs for all the sizes, what you can do is leave the GTINs empty.
It’s better to leave it empty rather than filling it up with incorrect values.
By leaving it empty, you will only get a warning but your products will not get disapproved in the Merchant Center.
Secondly, don’t submit GTINs in the restricted ranges.
Certain ranges of GTINs are restricted and should not be used by manufacturers or retailers. These restricted ranges have the prefixes 02, 04, or 2, and they are reserved for internal use by certain organizations.
We have a blog where we have mentioned all the GTIN errors and explained in detail about the solutions which you can try out and fix these errors ASAP.
GTIN Blog - https://blog.adnabu.com/google-merchant-center/how-to-fix-gtin-errors/
Best Regards,
Harsh
Team Adnabu