Nice model. Yup it's straight forward to make and upload a gradient background image, or use the supplied ones.
But it'd be easier to experiment with background treatments if it was configurable from within the Sketchfab editor.
Plus, my next request will be to make the background gradient get and set'able from the API
Here's Biodigital Human letting you configure gradient thru a UI, and it can also be set from API. Meaning we can re-treat the model based on what the player demands. eg. like a darkened night mode.