First of all, please log in to your account, as described in workspaces.
You can import files from GitHub public repository or public gist just by clicking on "Import from GitHub" at the bottom side of the workspace list (left column).
You can save all your files in active workspace as a gist. You have to switch to "Workspaces online" and provide your credentials for GitHub. Then you can use the feature "Save workspace as gist".
You will need two information: your user name for GitHub and your personal access token, which you have to generate here. Please grant access to gists and repositories when creating token. Your credentials are stored in your online account, so you have to provide it only once.