Gratis and Libre are two terms that are in some way tied to one other, although they relate to different matters. They are also often treated as one and the same, or used interchangeably. Generally, in contrast to Gold, Green and Hybrid OA, they do not describe forms of publication, but define the attributes of an article published in OA. Therefore, you can find instances when an article is described jointly as Gratis Open Access, or Gratis Gold/Green Open Access, etc.
The differences between these two forms are well described by Peter Suber: “Gratis” access is free of charge. “Libre” access is free of charge and free for some kinds of further use and reuse. Gratis access is compatible with an all-rights-reserved copyright, which allows no uses beyond fair use (or the local equivalent). Libre access is not compatible with an all-rights-reserved copyright, and presupposes some kind of open license permitting uses not permitted by default. As I’ve sometimes put it, gratis removes price barriers alone and libre removes price barriers and permission barriers.