New Twiddle

components.child\.hbs

<GrandChild ...attributes />

components.grand-child\.hbs

<button {{on "click" this.onButtonClick}} ...attributes>Click Me</button>

components.grand-child\.js

import Component from '@glimmer/component';

export default class extends Component {
  onButtonClick({ target }) {
    const event = new CustomEvent('specialmessage', {
      bubbles: true,
      detail: {
        message: 'hi ma!',
      }
    });
    target.dispatchEvent(event);
  }
}

components.parent\.hbs

<Child {{on "specialmessage" this.onSpecialMessage}} />

components.parent\.js

import Component from '@glimmer/component';
import { action } from '@ember/object';

export default class extends Component {
  @action
  onSpecialMessage(e) {
  	alert(e.detail.message);
  }
}

styles.app\.css

body {
  color: white;
  margin: 12px 16px;
  font-family: 'Helvetica Neue', Helvetica, Arial, sans-serif;
  font-size: 12pt;
}

templates.application\.hbs

<h1>Welcome to Ember</h1>
<br>
<br>
<Parent />
<br>
<br>

twiddle\.json

{
  "version": "0.17.1",
  "EmberENV": {
    "FEATURES": {},
    "_TEMPLATE_ONLY_GLIMMER_COMPONENTS": false,
    "_APPLICATION_TEMPLATE_WRAPPER": true,
    "_JQUERY_INTEGRATION": true
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/3.5.1/jquery.js",
    "ember": "3.18.1",
    "ember-template-compiler": "3.18.1",
    "ember-testing": "3.18.1"
  },
  "addons": {
    "@glimmer/component": "1.0.0"
  }
}