Gracia and Graeria are two sisters of Gwalchmei (= Gawain) in the 14th cenury fragmentary Welsh text known as The Birth of Arthur, from Llanstephan MS 201, edited and translated by J. H. Davis in Y Cymmrodor Vol. 24 (1900), pp. 249–264. The Welsh sentence in which they are mentioned runs:
Ac Uthyr [a beris i Leu vab Kynvarch] i phriodi a phlant [a gowsant nit amgen no] deu vab Gwa[lchmei a Medrawt a thair mer]chet Gracia G[raeria Dioneta. ...]
The English translation runs:
Now Uther caused Lleu, the son of Cynvarch, to marry her, and they had children, two sons, Gwalchmai and Medrawd, and three daughters, Gracia, Graeria, and Dioneta.
The work, which contains the story of the sword in the stone, is obviously at least partially based on the French History of Merlin, attributed to Robert de Boron. The corresponding material in this romance, in Alexandra Micha’s critical edition is:
... et de ceste fille que il donna le roi Loth eissi Mordrez et mes sires Gauvains et Gareés et Gaheriez, ...
The Enlglish translation with the proper names normalized is:
... and from that daughter whom he [Uther] gave to King Loth issued Mordred and Sir Gawain and Guerrehet and Gaheriet, ...
The two sisters are evidently corruptions by a Welsh translator or adapter or later copier of Gawain’s two brothers in the French text.