Changed to addEventListener as per Kuilin

genocideManArrowKeys.user.js

// ==UserScript==
// @name        Genocide Man Arrow Keys
// @namespace   https://gist.github.com/JulianWebb
// @version     0.2
// @author      Pongles
// @description Adds the ability to navigate the comic using just the left and right arrow keys
// @icon        https://www.genocideman.com/wp-content/uploads/2015/08/cropped-genoman-logo-icon-32x32.jpg
// @match       https://www.genocideman.com/*
// @grant       none
// ==/UserScript==

(function() {
  'use strict';
  
  console.log('Genocide Man Arrow Keys is working!')
  document.addEventListener("keydown", event => {
    switch (event.key) {
      case 'ArrowLeft':
        let prevPage = document.querySelector("a[rel=prev]");
        prevPage.click()
        break;
      case 'ArrowRight': 
        let nextPage = document.querySelector("a[rel=next]");
        nextPage.click();
        break;
      default: return;
    }
  });
  
})();