There is a significant amount of content on Medium that's readable without an account. I think, if the goal is to get eyes on your content, Medium's recommendation of your stuff to other people is a great way to achieve that.
To me, this engagement model is parasitic and suggests that their valuation/monetization strategy is "eyeballs". Not sure this necessarily helps writers but it certainly helps Medium to drive sign-ups with free content.
Gists do not require an account/login for readers AFAIK.