As you are probably aware at this point Google Shopping/Merchant Centre is going to start enforcing GTIN/EAN/UPC numbers from 16th of May 2016. If you are not sure what does it mean we hope you will find some answers below.
So GTIN is a fancy name for an umbrella like concept accommodating and normalising various Universal Number notations such as ISBN (Books), UPC (USA version of a barcode), EAN (European Version of a barcode) and more...
Basically, GTIN is a 14 digits number, it can be an EAN number with pre-appended 0 so it becomes 14 digits long, or it can be a UPC number with two leading 00s in front.
From 16th of May Google will start enforcing GTIN numbers for all branded items, by branded items they mean items with a brand which is sold by other sellers too. It's not clear how many sellers need to sell a brand to be considered as a cross-sold brand and not a custom one but you can probably fairly assume it based on how popular each brand is.
Therefore you won't need GTINs for custom branded items, made to order items etc. but you will need it for all other products.
The issue here is that there is no global DB to get GTINs, even Google itself doesn't have one which is probably why they want you to help them build it over the time.
The good news though is that we at Feed Optimise have worked hard over past weeks to build such a DB so we can help our customers to be compliant with the latest Google specification.
As far as the state of our GTIN's DB is concern, we are fairly confident in the coverage for sectors such as Home Appliances, Consumer Electronics and are working very hard to cover all others too so as long as you can provide us with valid MPN and Brand we hope to be able to tell you what is a GTIN for that item in return, therefore allowing you to be listed in Google Shopping even if you don't have GTIN yourself.
If you have any questions please feel free to contact us.