How to use grapesjs to Ruby on Rails 6 app?

GrapesJS has nothing to do with your back-end stack and seems like you just didn't use the CSS in examples, eg:

#gjs {
  border: 3px solid #444;
}

/* Reset some default styling */
.gjs-cv-canvas {
  top: 0;
  width: 100%;
  height: 100%;
}