Incorrect (traditional action attribute):

// app/components/button.gjs
import Component from '@glimmer/component';
import { action } from '@ember/object';

class Button extends Component {
  @action
  handleClick() {
    this.args.onClick?.();
  }

  <template>
    <button onclick={{this.handleClick}}>
      {{@label}}
    </button>
  </template>
}