It says it right on the homepage. Twice. Once for people, once for organisations. It’s right there in green: “BEST (SCORED OUT OF 100)”. And if you go into any of them, you see a score like N/100.
Found the methodology page, and it clarifies it goes from -100 to 100.
https://goodindex.org/methodology#:~:text=How%20Scoring%20Wo...