This is an accepted solution.
This is failry common scenario for new stores. Since you don't have ratings (stars) and reviews, it throws this warning. This is not an error, so you shouldn't be too worried about it.
The GTIN, MPN, UPC, ISBN, EAN are all identifiers usually assigned by the manufacturer.
If you don't have a GTIN for a product, then you can request it from the manufacturer.
Hope this helps!