Sample Salesforce web-to-lead form that captures Adwords GCLID in a hidden form field & Google Analytics Visitor ID in another hidden form field. Make sure to replace xxxxx, yyyyy, zzzzz with your specific Salesforce field values. Include your Google Analytics Tracking ID. Create a User Scope custom dimension in your Google Analytics property Admin under Custom Definitions > Custom Dimensions and update the dimension number on line 15.

## salesforce_adwords_google_analytics_integration_web-to-lead_form_example.html
<!DOCTYPE html>
<html>
<head>
<META HTTP-EQUIV="Content-type" CONTENT="text/html; charset=UTF-8">

<script>
(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','https://www.google-analytics.com/analytics.js','ga');

ga('create', 'UA-XXXXX-Y', 'auto');
ga(function(tracker) {
  var clientId = tracker.get('clientId');
  tracker.set('dimension100',clientId);
});
ga('send', 'pageview');
</script>

</head>

<body>
<form action="https://www.salesforce.com/servlet/servlet.WebToLead?encoding=UTF-8" method="POST">

<script>
        window.onload = function getGclid() {
             document.getElementById("xxxxx").value = (name = new
        RegExp('(?:^|;\\s*)gclid=([^;]*)').exec(document.cookie)) ?
        name.split(",")[1] : "";
            document.getElementById("zzzzz").value = (name = new
        RegExp('(?:^|;\\s*)_ga=([^;]*)').exec(document.cookie)) ?
         name.split(",")[1].split(".").slice(2).join(".") : "";
        }
        </script>

<input type=hidden name="oid" value="yyyyy">
<input type=hidden name="retURL" value="http://">

<label for="first_name">First Name</label><input  id="first_name" maxlength="40" name="first_name" size="20" type="text" /><br>

<input id="xxxxx" name="xxxxx"  type="hidden" /><br>
<input id="zzzzz" name="zzzzz"  type="hidden" /><br>

<input type="submit" name="submit">
</form>

<script type="text/javascript">
function setCookie(name, value, days){
    var date = new Date();
    date.setTime(date.getTime() + (days*24*60*60*1000));
    var expires = "; expires=" + date.toGMTString();
    document.cookie = name + "=" + value + expires;
}
function getParam(p){
    var match = RegExp('[?&]' + p + '=([^&]*)').exec(window.location.search);
    return match && decodeURIComponent(match[1].replace(/\+/g, ' '));
}
var gclid = getParam('gclid');
if(gclid){
    var gclsrc = getParam('gclsrc');
    if(!gclsrc || gclsrc.indexOf('aw') !== -1){
	    setCookie('gclid', gclid, 90);
	}
}
</script>
</body>
</html>
	<!DOCTYPE html>
	<html>
	<head>
	<META HTTP-EQUIV="Content-type" CONTENT="text/html; charset=UTF-8">

	<script>
	(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','https://www.google-analytics.com/analytics.js','ga');

	ga('create', 'UA-XXXXX-Y', 'auto');
	ga(function(tracker) {
	var clientId = tracker.get('clientId');
	tracker.set('dimension100',clientId);
	});
	ga('send', 'pageview');
	</script>

	</head>

	<body>
	<form action="https://www.salesforce.com/servlet/servlet.WebToLead?encoding=UTF-8" method="POST">

	<script>
	window.onload = function getGclid() {
	document.getElementById("xxxxx").value = (name = new
	RegExp('(?:^\|;\\s)gclid=([^;])').exec(document.cookie)) ?
	name.split(",")[1] : "";
	document.getElementById("zzzzz").value = (name = new
	RegExp('(?:^\|;\\s)_ga=([^;])').exec(document.cookie)) ?
	name.split(",")[1].split(".").slice(2).join(".") : "";
	}
	</script>

	<input type=hidden name="oid" value="yyyyy">
	<input type=hidden name="retURL" value="http://">

	<label for="first_name">First Name</label><input id="first_name" maxlength="40" name="first_name" size="20" type="text" /><br>

	<input id="xxxxx" name="xxxxx" type="hidden" /><br>
	<input id="zzzzz" name="zzzzz" type="hidden" /><br>

	<input type="submit" name="submit">
	</form>

	<script type="text/javascript">
	function setCookie(name, value, days){
	var date = new Date();
	date.setTime(date.getTime() + (days2460601000));
	var expires = "; expires=" + date.toGMTString();
	document.cookie = name + "=" + value + expires;
	}
	function getParam(p){
	var match = RegExp('[?&]' + p + '=([^&]*)').exec(window.location.search);
	return match && decodeURIComponent(match[1].replace(/\+/g, ' '));
	}
	var gclid = getParam('gclid');
	if(gclid){
	var gclsrc = getParam('gclsrc');
	if(!gclsrc \|\| gclsrc.indexOf('aw') !== -1){
	setCookie('gclid', gclid, 90);
	}
	}
	</script>
	</body>
	</html>