AFAIK the GWO approach is nothing more than showing alternative versions of pages to measure preference of your visitors based on bounce etc. But the alternative page versions should display to humans only. That's something you should be doing in your code. And it's a bit more complex to do with landing pages from SERPs.
A simple method is to use a cookie to differentiate between bots and humans. Ensure if the cookie is not present your server sends the original version of the page.
Now if it's a landing page you need to start checking HTTP headers to identify if it's a bot, as the cookie is of no use - browsers won't send the cookie with the first request because they don't have it.