You will generally utilize Gametime through the Timeboard prefab, which you should add to your scene. This is required for automatic operation of Conditions and Quests.
The Timeboard component has a Gametime class, where you can set up the options you'll be using for your project.
Create a Prefab Variant for your version of Timeboard to avoid accidental overwriting if the prefab which comes with the Game Modules package is ever updated.
The DatePart Enum is used in many methods, and contains Second, Minute, Hour, Day, Week, Month, Season, and Year.
The value of gameTime is will be 0 at the start of the first day of the startingYear.
As you update the other starting values in the Inspector, you'll notice the value of gameTime will change.