To sell your products on Google and other similar stores, you need to buy GTIN codes at GS1; it is crucial to purchase a GTIN prefix from gs1 and generate barcodes using that prefix (some exception exists for custom made products).

Once the prefix has been bought, you can upload them into Kodbar through a CSV file. Kodbar will assign them automatically. 

You can buy the GTIN barcodes directly from GS1 or resellers. https://www.gs1.org/standards/barcodes