This is to be able to inspect the url query string and send the details to Google Analytics through Google Tag Manager

ga_url_tracking_via_gtm.js

<script type="text/javascript">

/*eslint-env browser*/
/*global ga*/

(function(i,s,o,g,r,a,m){i['GoogleAnalyticsObject']=r;i[r]=i[r]||function(){
(i[r].q=i[r].q||[]).push(arguments)},i[r].l=1*new Date();a=s.createElement(o),
m=s.getElementsByTagName(o)[0];a.async=1;a.src=g;m.parentNode.insertBefore(a,m)
})(window,document,'script','//www.google-analytics.com/analytics.js','ga');

ga('create', '<API PROPERTY>', 'auto');
ga('send', 'pageview');


(function(u) {

    'use strict';

    function ucfirst(string) {
        return string[0].toUpperCase() + string.slice(1);
    }

    function parseUrlQueryParams(string) {
        var query = {};
        var a = string.split('&');
        var max = a.length;

        for (var p = 0; p < max; p++) {
            var b = a[p].split('=');
            query[decodeURIComponent(b[0]).trim()] = decodeURIComponent(b[1]).trim();
        }

        return query;
    }

    if (u !== '') {

        var URL = parseUrlQueryParams(u);
        var needles = ['category', 'action', 'label', 'value', 'nonInteraction'];
        var mustHave = ['category', 'action'];
        var eventDetails = {'hitType': 'event'};
        var validGaUrl = true;
        var maxMustHave = mustHave.length;
        var maxNeedles = needles.length;

        for (var m = 0; m < maxMustHave; m++) {
            if (typeof URL['event_' + mustHave[m]] === 'undefined') {
                validGaUrl = false;
            }
        }

        if (validGaUrl === true && typeof ga === 'function') {

            for (var n = 0; n < maxNeedles; n++) {
                if (typeof URL['event_' + needles[n]] !== 'undefined') {
                    var eventName = 'event' + ucfirst(needles[n]);
                    eventDetails[eventName] = URL['event_' + needles[n]];
                }
            }

            ga('send', eventDetails);
        }
    }

}({{url_query}}, ga));

</script>

on line 14 i'd trim both the key and the value just to help with typos that might creep in

is parseQuery() expected to operate on document.URL

If so, it's going to miss the first parameter in a query string. Would need to slice the URL from the "?" before running thru parseQuery()

And wrap the whole thing in an anonymous function so as to manage var scoping

No, on GTM you can get the query string, and they provide is without the leading "?"