Strange.
Let’s try finding out which specifically is the issue.
Can you try disabling all plugins except GP Premium? To see if it’s a plugin issue.
I see that you’re using a child theme as well. Can you try temporarily deactivating the child theme and use just the parent theme for now? To see if it’s a template issue.
Let us know.
Note: if you’re using a custom child theme template that’s missing a wp_head hook for the product pages, it makes sense why the wp_head hook doesn’t work because there’s no wp_head to hook things in.