Simple script that will add a way to change WordPress Gutenberg editor styles based on template chosen for the post or page.

add_template_to_gutenberg_editor.js

/**
 * License: GPLv2
 * Created by Roger Los  (rogerlos.com)
 * https://github.com/rogerlos
 */
jQuery( function ( $ ) {
  
  /**
   * Some values needed for this to work. The actual select is not set in this constant as jQuery won't
   * catch its value changing, in my experience...
   * 
   * CONTROL_WRAPPER - jQuery selector; Required, Gutenberg wrapper containing the template dropdown
   * APPEND_TO       - jQuery selector; Required, Where you would like to append your template class in the editor
   * CLASS_PREFIX    - String; Optional, will add a prefix before your template class
   */
  const CONTROL_WRAPPER = '.editor-page-attributes__template' );
  const APPEND_TO = '#editor';
  const CLASS_PREFIX = 'mytheme-template-';
  
  // keeps track of the chosen template
  let CURRENT = '';

  /**
   * After the editor has loaded:
   * 1. Find the template select control
   * 2. Add class to your chosen DOM object
   * 3. Watch for new template selection
   */
  $(window).on('load', function() {

    let $template = $( CONTROL_WRAPPER ).find( 'select' );
    
    editor_add_class( $template.val() );

    $template.on( 'change', function() {
       editor_add_class( $(this).val() );
    });
  });
  
  /**
   * Adds template class to APPEND_TO.
   *
   * @param {string} value - New template file name, including extension
   */
  function editor_add_class( value ) {
    
    let $wrapper = $( APPEND_TO );

    // make sure value is set
    value = typeof( value ) === 'undefined' ? '' : value;

    // dancing: the old template class is removed and the new one added
    $wrapper.removeClass( CURRENT );
    CURRENT = editor_class( value );
    $wrapper.addClass( CURRENT );
  }
  
  /**
   * Mashes together chosen prefix with the template filename after stripping
   * the "php" extension.
   *
   * @param {string} template - Value gleaned from template select control
   * @returns {string}
   */
  function editor_class( template ) {
    
    let regex = /\.php/i;
    
    template = template.replace( regex, '' );
    
    return CLASS_PREFIX + template;
  }
} );

gutenberg_template_class.php

<?php
/**
 * License: GPLv2
 * Created by Roger Los (rogerlos.com)
 * https://github.com/rogerlos
 *
 * The code below assumes use in a theme, and that the JS file above is within theme's 'js' directory.
 * Adjust to suit if you're working within a plugin.
 */

/**
 * Adds JS to admin.
 */
function add_template_class_to_gutenberg() {
  
  wp_enqueue_script( 
    'gutenberg-template-class',
    get_stylesheet_directory() . '/js/add_template_to_gutenberg_editor.js',
    [ 'jquery' ],
    '1.0',
    true
  );
}

/**
 * Adds CSS sheet to editor.
 */
function add_editor_css() {
  
  wp_enqueue_style( 'mytheme-gutenberg-css', get_stylesheet_directory() . '/css/sample_editor.css' );
}

// You could potentially add the JS as a block asset as well, but I haven't tried it.
add_action( 'enqueue_block_assets', 'add_editor_css' );
add_action( 'admin_enqueue_scripts', 'add_template_class_to_gutenberg' );

sample_editor.css

/* Here's an example where different editor widths are set for different templates */
/* All of this theme's theoretical templates use a 1000px width, except a sidebar  */
/* template 'page-with-sidebar.php'                                                */

.wp-block {
  max-width: 1000px;
}

.mytheme-template-page-with-sidebar {
  .wp-block {
    max-width: 600px;
  }
}