@MendesSilva
I know that you are able to create a new Amazon product without a pre-existing GTIN listing on Amazon using the Amazon sales channel. As to whether it would work if your product doesn't have one, I can't say as our products all had UPC's, so we never encountered that situation.
We no longer use the Amazon sales channel itself - we use Sellbrite now (there were other marketplaces we needed to connect to that don't currently have a Shopify sales channel). With Sellbrite, you can create new listings for a product that already exists on Amazon, but you cannot create a new Amazon product. So when we start carrying a new item that doesn't exist on Amazon yet, we have to create it from within our seller central account on Amazon and then connect our Shopify listing from Sellbrite to the newly created product.
I am also not familiar with Orderdesk, but looking at the documentation just now, I don't see where creating new products or even creating new listings for existing products is an available feature. it appears that you would need to create the products and/or set up your listings using another method, and Orderdesk will simply keep the inventory synced between Amazon/Shopify once they are all created.
