How Do You Add A Form on a Page to Gate the Content?
Depending on your Craft License you could make use of a login form as described inside the docs here.

If you got the loginform working, simply use {% requireLogin %} on the site template (i.e. inside the twig template file) that should have restricted access.

If your goal is to just provide an E-Mail without a password you would need to tinker around a bit and maybe just write up a small plugin. To get startet I suggest reading this and this. Out of the box this is not possible as far as I know.
I would advise not just using an E-Mail for validation as ab@c.de for example is a valid E-Mail, though unlikely that it is real. Some sort of login/auth would provide a more solid experience and enforces that the user is real and has validated their E-Mail with the CMS (as craft can send a validation link to given E-Mail after registering with the site).