In the One Piece series, "Heaven's Door" refers to a technique that is attached to the Ato Ato no Mi Devil Fruit power. The ability belongs to Giolla, a member of the Donquixote Pirates. This technique is officially titled "Heaven's Door Art" in the Funimation adaptations of the anime.
Giolla's Heaven's Door Art is the consequential ability from her Devil Fruit: turning people and objects into abstract art. In so doing, Giolla turns herself into a living piece of art when using this ability, hence changing her appearances and probably her physical characteristics. This is pretty much visually appealing in the anime since it's depicted by swirling, shapeless patterns and bright colors that represent the abstract nature of her powers.
During the Dressrosa arc, one of the techniques most often used is the "Heaven's Door Art," mainly with regard to Giolla's fights against the Straw Hat Pirates. Notably, there is when she tries using that very ability in order to counter an attack from Nami. Still, this technique cannot do anything against the naked power of the Gaon Cannon of the Thousand Suns, which seals Giolla's defeat.
Speculations by some fans even claim that "Heaven's Door Art" by Giolla may be a form taken after awakening her Devil Fruit powers. The thing is, it does affect the space around her, much like other awakened Paramecia-type Devil Fruits usually do. However, it has not been officially confirmed in the series yet.
There should not be any confusion between the "Heaven's Door" in One Piece and the Stand ability of the same name that appears in JoJo's Bizarre Adventure, even though both relate to transformative powers themselves.
The method will first appear during the 17th season of the One Piece anime, covering episodes adapting content from the Dressrosa arc. Amidst the many arcs that make up the series, this is one of the most notable for its number of Devil Fruit powers and their variety in their uses both in and outside of combat.