Welcome to the forum 
Every library you install comes with examples, select File > Examples from the menus you will find G4P in the contributed libraries section. You can also look at the G4P guides on my website.
The easiest solution is probably to install the GUI Builder using Tool > Add tools from the menus. GUI Builder provides a graphical interface to design and create G4P GUIs. There are also GUI Builder videos on my website.
Please note that the G4P library the and GUI Builder tool are feature rich and you will not master them overnight. I strongly recommend that start small and experiment with them first. When you are confident that you can use them then try and create your project.