Getting your products grouped in Google Shopping is not a great position to be in. Mainly because some competitors are not being grouped and might hold an advantage over grouped items.
The grouping is done dynamically based on product identifiers, user history, current actions, search terms, data information and other factors. But in my opinion mainly due to the identifier.
One of the most contributing reason is the identifier. When you submit a product and you add MPN, GTIN and Brand than you are telling Google in a very accurate way what your product is. Because there are so many products in Google Shopping results that are exactly the same, Google tries to merge them in one compare section. This to better show the products to the customers.
As much as we understand the reason, it is not well implemented.
There are also other factors, but these are not known and we would not be able to control this anyway.
Don’t change the GTIN just for sake of being un-grouped. This can result in your products being disapproved. Google is currently forcing merchants that have products listed in the designated brands to add GTIN values.
Report any products that you believe have no GTIN values by submitting a violation to Google. Also a good idea is to report any other violations the competitor is doing. I found this very effective. It might take upto a couple of months for them to make any changes but we all have to play by the same rules. But currently how the automated system is working. It’s an unfair system.