HotJar is a third-party analytics tool that generates a heat map of visitors' behavior on the site. Their tracking code should be placed somewhere on the home page.
Gutensite Control Panel --> Content --> Design --> Customize Design
Typically, any of the fields that offer the editor for text can also accept code, e.g. Text Area. Switch to <code view>, then paste the code. Sometimes, you'll see another field called "Allow raw HTML". Make sure that box is checked, otherwise scripts will be ignored.