One simple option is passing an URL parameter in the email link and then, in the landing page, checking whether that parameter is valid. It requires minimum time and effort to implement.
The email link would be something like http://www.example.com?paid_user_sfdc={{lead.token_for_paid_user}}
The landing page would have a hidden field to read the URL parameter
The last step is defining a short jQuery script redirects to the relevant link
<****** type="text/javascript" src="/js/public/jquery-latest.min.js"></*****>
<***** type="text/javascript">
// set no conflict mode for jquery
var $jQ = jQuery.noConflict();
$jQ(document).ready(function(){
// compare the hidden field with the expected value
// if matches, redirect to the intended gated content
// window.location.href = 'http://www.example.com/gated.html';
// otherwise send to another page
// window.location.href = 'http://www.example.com/';
});
</******>
Where you see ****** use "script"