the Sites in our library are kept up to date and yes we provide support with any GP related issues. If there is something that isn’t working for you then we can assist.
To note we do not recommend installing a GP Site on a pre-built website, as the results may not be as expected. None of your existing content will be removed, but things such as logos, menus and widgets will get swapped, it will also install the Sites pages and Custom CSS along with any plugins. So conflicts are possible.
If you’re going to attempt it then make sure you have a backup or are working on a non production copy of the site. The alternative is to install the Site on a clean build and import your content.