This userscript will add an 'Install' link to all userscript files (which end with .user.js by necessity).

gistUserScriptInstallLink.user.js

// ==UserScript==
// @name			Gist UserScript Install Link
// @author			Erik Vold
// @namespace		gistUserScriptInstallLink
// @include			https://gist.github.com/*
// @version			0.9
// @license			GPL version 3 or any later version; http://www.gnu.org/copyleft/gpl.html
// @description		This userscript will add an 'Install' link to all userscript files (which end with .user.js by necessity).
// @downloadURL https://gist.github.com/surkin/ffbcaeb5afa6f155a8d6/raw/e62b203f2ef9233b1894552b7b53408115434051/gistUserScriptInstallLink.user.js
// @grant none
// ==/UserScript==

(function (doc) {
  var userscripts = doc.evaluate("//a[contains(substring(@href,string-length(@href)-9),'.user.js') and @original-title='Raw']", doc, null, 6, null),
    userscript, raw;
  for (var i = userscripts.snapshotLength - 1; i > -1; i--) {
    userscript = userscripts.snapshotItem(i);
    userscript.innerHTML = 'install';
    raw = userscript.cloneNode(false);
    raw.innerHTML = 'raw';
    raw.href = raw.href + "?";
    userscript.parentNode.insertBefore(raw, userscript);
  }
})(document);