Use post meta in gutenberg

index.js

// https://make.wordpress.org/core/2020/03/02/general-block-editor-api-updates/
// https://github.com/WordPress/gutenberg/tree/trunk/packages/core-data

import {
  PanelRow, TextControl,
} from '@wordpress/components';
import { useSelect } from '@wordpress/data';
import { useEntityProp } from '@wordpress/core-data';
import { PluginDocumentSettingPanel } from '@wordpress/edit-post';
import { registerPlugin } from '@wordpress/plugins';

function CustomMetaPanel() {
  const postType = useSelect(
    (select) => select('core/editor').getCurrentPostType(),
    [],
  );

  const [meta, setMeta] = useEntityProp('postType', postType, 'meta');
  const metaValue = meta.customMeta;
  const updateMetaValue = (newValue) => {
    setMeta({ ...meta, customMeta: newValue });
  };
  return (
    <PluginDocumentSettingPanel name="customMetaPanel" title="Custom meta">
      <PanelRow>
        <TextControl
          label="Custom meta"
          value={metaValue}
          onChange={updateMetaValue}
          className="custom-meta-field"
        />
      </PanelRow>
    </PluginDocumentSettingPanel>
  );
}

registerPlugin('plugin-name', {
  render: CustomMetaPanel,
  icon: '',
});

Thanks for this - was struggling to get this working.