It used to be here: https://about.gitlab.com/handbook/total-rewards/compensation/compensation-calculator/calculator/

But now it says: "The compensation calculator is available to all our team members through this url."

This is the last version available on the Internet Archive: https://web.archive.org/web/20201019140406/https://about.gitlab.com/handbook/total-rewards/compensation/compensation-calculator/calculator/