The Gamebuino library includes several examples for you to understand how to make games. They are heavily commented so you can understand what each line does. If you never ever programmed though, you should read the Foundation article from Arduino.
You should start to go through the basic examples first, and if you have any doubt about what a function does you'll find it in the Gamebuino Reference for Gamebuino specific stuff, or in the Arduino Reference for general C++/microcontroller things like conditional loops or data types.
Once you have installed the library (following the Getting started guide), you can open the examples directly from the Arduino software in File/Examples/Gamebuino. You can also find them on Gamebuino's GitHub in /libraries/Gamebuino/examples.
You can also get inspired by games other people did, you'll find them in the Games Gallery.
You don't even need a Gamebuino to start developing games, you can use one of the Emulators!
Samples that may or may not use the official library but are designed to demonstrate the hardware's capabilities.