Fetch polyfill (https://github.com/github/fetch) stripped down to provide GM_fetch via GM_XHR.

GM_fetch.js

(function() {
  'use strict';

  var support = {
    blob: 'FileReader' in self && 'Blob' in self && (function() {
      try {
        new Blob();
        return true
      } catch(e) {
        return false
      }
    })(),
    formData: 'FormData' in self,
    arrayBuffer: 'ArrayBuffer' in self
  }

  function headers(xhr) {
    var head = new Headers()
    var pairs = xhr.getAllResponseHeaders().trim().split('\n')
    pairs.forEach(function(header) {
      var split = header.trim().split(':')
      var key = split.shift().trim()
      var value = split.join(':').trim()
      head.append(key, value)
    })
    return head
  }

  self.GM_fetch = function(input, init) {
    return new Promise(function(resolve, reject) {
      var request
      if (Request.prototype.isPrototypeOf(input) && !init) {
        request = input
      } else {
        request = new Request(input, init)
      }

      var xhr = new GM_XHR()

      function responseURL() {
        if ('responseURL' in xhr) {
          return xhr.responseURL
        }

        // Avoid security warnings on getResponseHeader when not allowed by CORS
        if (/^X-Request-URL:/m.test(xhr.getAllResponseHeaders())) {
          return xhr.getResponseHeader('X-Request-URL')
        }

        return;
      }

      xhr.onload = function() {
        var status = (xhr.status === 1223) ? 204 : xhr.status
        if (status < 100 || status > 599) {
          reject(new TypeError('Network request failed'))
          return
        }
        var options = {
          status: status,
          statusText: xhr.statusText,
          headers: headers(xhr),
          url: responseURL()
        }
        var body = 'response' in xhr ? xhr.response : xhr.responseText;
        resolve(new Response(body, options))
      }

      xhr.onerror = function() {
        reject(new TypeError('Network request failed'))
      }

      xhr.open(request.method, request.url, true)

      if (request.credentials === 'include') {
        xhr.withCredentials = true
      }

      if ('responseType' in xhr && support.blob) {
        xhr.responseType = 'blob'
      }

      for (let header of request.headers) {
        xhr.setRequestHeader(header[0], header[1]);
      }

      xhr.send(typeof request._bodyInit === 'undefined' ? null : request._bodyInit)
    })
  }
  self.GM_fetch.polyfill = true
})();