Make sure you have installed the Shopify Plugin
1. Add a product to Shopify in the usual way then change the template of the product to product.G3d and add the g3D product ID in the barcode field - the normal add to cart button will now change to "Customise & Buy"
2. For Print On Demand products simply create product in shopify leave it on the standard product template but include in the barcode field "POD xxxxxxxxx" were xxxxxx is the POD id form G3D. This will then auto create a print job in the Gateway 3D Workflow Manager everytime the job is ordered