You can retry importing the site.
RE this:
Is it a case of importing a site from the site library (“Existing Site Import Detected”) – and if I do will this overwrite the articles I’ve already created?
No it will not overwrite your existing content.
However we strongly recommend you make a backup of your site or create a staging site to do the import.
To make sure you get the correct design – during the import process you must Import the Demo Content see here for more info:
https://docs.generatepress.com/article/using-the-site-library/
You then need to delete the demo posts and pages that you don’t require.