Should content be gated? If so, what's the basis for distinguishing gated vs non gated content?

It entirely depends on the goals of your marketing efforts. For example, do you want many downloads or views or clicks on a piece of content or do you want customers contacting you or an increase in sales revenue?  If you want potential leads and are into B2B content marketing, then use gated content. That way you can collect a lot of relevant information about your audience. Now, here is a catch. Your content actually has to be good enough for a potential lead to fill out a form or any details that you want. And the forms have to be short and crisp. Ungated content however, will give you more downloads, clicks and views but does not necessarily generate leads. Both forms have their pros and cons. Again, content marketing is about trying and testing what works and what doesn't.  

The main purpose of content gating is lead generation. The gated content is any content that requires a user to provide contact information such as a form completed with their email id, contact details, preferences, etc.. The ungated content is the one gives out information without asking for the reader's details such as blog posts, Facebook pages, posts and more.