Common issue.
Some resellers go buy GTINs, and that messes up the plans of any CSEs who want the items to have a common GTIN, the whole point.
Don't buy GTINs if you're not the OEM, don't scrape them either, those could be fake.

There is a process for items without assigned GTINs, see "Products Without a GTIN" here:
[support.google.com...]

In this case the OEM says they haven't assigned a GTIN but Google is acting as if they have by blocking some of the products in our feed. We have submitted a brand and MPN for all products in the feed. This is not enough for 5 of the products. The page your kindly linked says that all we have to do is include a band and MPN for each product.

I suspect that one of our competitors may have assigned a GTIN to the product or the OEM won't tell us the GTIN. Is there any way I can find out what this number is? On the page you linked it says there is a way, by finding the Details section on the compare prices page. I can't see that anywhere. Anybody know where the GTIN is displayed for products in Google Shopping?

Here: [support.google.com...]

It says:
~~~~~~~~~~~
Search for the product on Google Shopping. For some countries, if you search for the product, the Shopping ad will have a link to compare prices. Click this link, and then look for the GTIN under the Details section.
~~~~~~~~~~~

I took a screenshot, but don't know how to upload it here. :-(

I'll go see if I can message it to you...

I can't figure out how to attach a file, I'm a genius. :-)

Go to Google Shopping, search for "Learn & Master Guitar", find a product that is sold by many sellers, click on "Compare prices from XX stores", scroll up a hair, look for "details" link, click it, then look for GTINs and Part Numbers...

Good luck!

@RhinoFish - Thanks for that. The results page with the products in question don't show a details link. Shame, because at least then I could have found out who the GTIN belonged to.

I don't think I'm going to have any choice but to use my own GTIN. I don't want to and it voids the whole point of a GTIN but I can't see any other options at the moment.

In the end I had to message Google Merchant Center support to try to resolve the problem. They have now removed the error from one of the two products that were banned for not having a GTIN. Google fixing the problem has had a side effect upon the product that is now not excluded. When you go into Google shopping tab when you search for the product in question, Google now have this product and another reseller's entirely different product confused as the same product. You have the product title from the other product sold by the other reseller, with the price and listing as being me.

All told I think Google have quite a big bug in Google merchant.

Spoke too soon. My best seller is now excluded from my Google Products feed again due to the GTIN bug.

All seems to have resolved itself for now. All of my feed is now live with zero changes from me. Whatever was causing the problem was fixed on Google's end. Affected us for the best part of a year.