There are 4 great ways to protect pages and content on your website.
1) PDF Password - If the content you are protecting is a simple PDF or could be saved as a PDF, then it often easiest to simply add a password to the PDF. Adobe Acrobat and other PDF software can add this password. PDF passwords help protect the file no matter where it might be shared. And, with a PDF password, you don't have to worry about doing any of the below options.
2) Hidden Page Trick - You can hide a page from navigation in the website editor. Here is a screenshot to guide you: https://www.screencast.com/t/WV6NDZ4PZJW If you then make the page name long and confusing such as galleryrestljument789 then the live link would look something like this: www.yourdomainname.com/galleryrestljument789 It is unlikely anyone will ever guess this long name, so the page could only be accessed by people with whom you share the link. This is not a rock solid solution, but it is a very good and very easy solution if the content you are protecting is not too sensitive. If you do go this route, you will also want to hide this page from search engines. To do so, go to pages->your page name -> SEO Settings -> and scroll to the bottom to "hide this page from search engines". Here is a screenshot to guide you: https://www.screencast.com/t/lHQBi7LIH5
The Hidden Page Trick is shown toward the very end of the below video.
3) Site Password
You can set a simple password that protects any page on the website to which you set the visibility to Site Password. Please see the below video to learn how to set a Site Password.
You can create members to your website and assign the visibility of certain pages to those member or members. Please see the below video to learn how to use Members to protects pages.