Hi there,
you would add it in the same way as if you were using a clean install of GP 🙂
You can use the GP Hook Element:
https://docs.generatepress.com/article/hooks-element-overview/
1. In the text area add your script.
2. Then set the Hook to: wp_header ( this is inside the websites <head>
)
3. Set the Display Rules > Location to the Entire Site.
If you’re using Google Tag Manager then that requires two codes, and this doc explains how:
https://docs.generatepress.com/article/implement-google-tag-manager/