That may be the worst code I've ever seen (and it offers no actual protection).
EDIT: It appears the post with the code I was referring to has been removed by the poster. That's for the best, thanks.
Zero security isn't really a starting point for anything, and that code encourages about 5 different worst practices.
Create a lead field LastGatedContentPassword. Add it to a form. Add a choice for Follow-Up URL that sends them to the asset on a match, to a failure page on a non-match. This stores the password on the server so it can't be read by the lead. You're adding only basic security, but it's a non-negligible barrier (while putting the password right in the HTML source is no barrier at all).
For a secure system, it would require an authentication server to be setup.
Questions like the following would need to be answered.
Doing a project like this would be much more than anyone could likely assist with in the forums, and you would likely need to engage a contractor to build it for you. It would take over 100 hours to build out something and begs the question, where you would use a Marketo Landing Page behind login credentials? It doesn't seem like the main purpose of a landing page and would be much better handled as part of the core functionality of your main website.
Idea is usually that a single passcode is distributed out-of-band (like at a tradeshow). That additional layer is more to establish that someone came in via a known route. If they game the system by guessing the URL, let alone having the asset and/or password sent to them, that isn't a major prob. But if you're going to have a passcode, at least don't put it on the page!