GTIN not available for products, Gshopping warning

Hello,

I’m creating a feed for my client who makes her own products, so there is no GTIN available. When uploading the feed to the Google Merchant Center, the warning ‘limited performance due to missing identifiers [gtin, mpn, brand] is visible. Brand is submitted, I just don’t have a GTIN. Is there a way I can set the GTIN identifier to ‘false’ in some sort of way (in order to let Google know that there is no gtin)?
Thanks in advance! ??