It is not currently possible to add new mockups from Gelato to BigCommerce for already published products. However, you can upload new images to your product when editing a product in BigCommerce. You can read more about how this can be done in BigCommerce's help center here.
You can download mockups by selecting the product in your store products page in Gelato and opening the design editor to edit the design. From the design editor, you can call up the preview and download the mockup images you fancy by right-clicking on them and saving them to your computer.
You can then upload them to BigCommerce.