When you create a new site, Stackbit creates and stores the code for that site in a repository under an account we manage. You can request to transfer ownership of that repo to your GitHub account.
This option is available in your site's settings. Click the cog icon in the top left of your screen to open the settings.
In the Connected Services section, choose Request Repo Transfer in the GitHub section.
GitHub will send an email notification to the primary email address on your GitHub account. Click the link in that email and you should see the repository appear within a few minutes.
The site settings will then be updated to reflect the new location.