Hello :)
Don't worry, once you understand how Firebase works, it becomes very easy to use !
You must create and link your Firebase database to your game first, follow the steps bellow.
Open Gdevelop → Open the template project → Go to Main menu scene → open the events tab → read the first comment (its a step by step + video URL on how to create and link your Firebase database to the game)
I also made this step-by-step tutorial video to make things even easier 😉