Hey @thejuniperstudio,
This is a good question. You can't create variations in the Prismic web app. And, unfortunately, @kris's suggestion won't work. The JSON structure is fundamentally different for variable Slices, and there's no support in the web app interface.
So, the workaround:
slice-machine-ui is meant to create both technology-specific Slice components in your codebase and a corresponding Slice model in your Prismic repo. (And Gatsby support is forthcoming.) However, it should be possible to use slice-machine-ui just to create the model in Prismic.
You should be able to create an sm.json file with
{ "apiEndpoint": "", "libraries": ["~/dummy"] }
And then create Slices in the UI.
You would need to make sure that your repository is on a Slice Machine repo. We also don't have a migration process, yet, so you would need to manually copy old Slices in Slice Machine.
You might have trouble with Storybook, since we haven't debugged the Storybook–Gatsby–Slice Machine integration, and that could be a blocker.
I hope this helps! Let me know if you have any questions about this.
Sam