With this script you are good to go for client-site A/B/n testing with Content Experiments. Add ?var=1 to preview the variation of the A/B test.

Google Analytics Content Experiments code for Google Tag Manager

<script src="//www.google-analytics.com/cx/api.js?experiment=YOUR_EXPERIMENT_ID"></script>
<script>
function getParameterByName(name) {
  name = name.replace(/[\[]/, "\\[").replace(/[\]]/, "\\]");
  var regex = new RegExp("[\\?&]" + name + "=([^&#]*)"),
      results = regex.exec(location.search);
  return results === null ? "" : decodeURIComponent(results[1].replace(/\+/g, " "));
}
var varID = getParameterByName('var');
if(! varID){ var chosenVariation = cxApi.chooseVariation(); }
else { var chosenVariation = varID; };
var pageVariations = [
  function(){
    ga("set","dimensionX","GTM-001_Control"),
    ga("send","event","AB-test","GTM-001 Name of the test","Control",{nonInteraction:1});
  },
  function(){
    ga("set","dimensionX","GTM-001_Variation_1"),
    ga("send","event","AB-test","GTM-001 Name of the test","Variation 1",{nonInteraction:1});
  }
];
$(document).ready(
  pageVariations[chosenVariation]
);
</script>