I've been looking for this solution, but from talking to marketo support - currently Marketo doesn't have a built in function to gate content. If someone who is not a known lead obtains the URL for one of the Marketo landing pages, they will be able to access that page. But in many scenarios - such as 'content download', you want leads to access the thank you page only upon submitting a form. I hope Marketo could build this soon.