It is not currently possible to add new mockups from Gelato to Squarespace for already published products. However, you can upload new and update existing images to your product when editing a product in Squarespace. You can read more about how to do this in the article Product images in Squarespace Help Center.
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 Squarespace.