Hi there,
you can use a site for our Site Library and you can see them all here:
https://generatepress.com/site-library/
And this doc explains how to us use it:
https://docs.generatepress.com/article/using-the-site-library/
However, they are designed to be used on New sites.
Its not impossible to add them to an existing site but there are somethings to note:
1. We advise you do this on a Staging site ( or clone of your existing site ) or at the very least make a backup of your site.
2. After importing a site it may contain the imported content, which will need deleted or updating.
3. Menus, Widgets, Logos will be imported with the site, so you will need to swap them back to your old sites.
4. If you have any Customizer > Additional CSS on your existing site, its best to copy this out to a notepad / IDE app for safe keeping as it may be overwritten when the site is imported.
It can take a bit of work depending on how your current site is setup