I think a general rule of thumb is that top-funnel and bottom-funnel content is not gated, while mid-funnel content is. Top-funnel content is pieces like blog posts, infographics, or cheat sheets. Mid-funnel content consists of meatier ebooks, whitepapers and reports. Bottom-funnel content includes demos, analyst reports etc.
At Marketo, we use all types of content in email marketing. But since we're emailing people in our database, when they click on a gated piece of content, they do not need to fill up a form again as we already have their information. So in that sense, every piece of content for them is ungated.