It is not currently possible to add new mockups from Gelato to Wix for already published products. However, you can upload new images to your product listings directly in Wix by selecting the product in the Store Products list and uploading new media files in the Images & videos section.
You can also upload a different image for each product option or variant in Wix.
You can read more about how you can achieve this in Wix's Help Center article Adding and Customizing Product Options.
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 in Wix as mentioned above.