Hey! I see you have troubles with game programming not the Editor itself.
I suggest to you look into existing game projects made in Godot 3 and understand how the things work.
Please look into these repos:
https://github.com/v-pukman-gd/beat-shooter-godot
https://github.com/v-pukman-gd/music-tiles
https://github.com/v-pukman-gd/guitar-hero-clone
After that you can start your project from scratch. It could be a challenge since the you need to develop gameplay by yourself. Editor is for rhythm mapping only. How it is processed in the game is the responsibility of the developer.
I hope it will help you!
Vic