I'm not sure I understand your questions. I haven't used GraphCMS so I don't know if they let you store HTML, but some CMS do, and yes that makes it harder to reuse your content in a context other than the web. I don't know, but I assume that you could probably make GraphCMS store HTML if you wanted to. The question of "can the CMS store HTML" seems orthogonal to the question of "Is it headless?" or "Does it have a GraphQL or REST API?"