Pa Gur/Gwr yv Y Porthaur?, or Arthur a Y Porthaur, or Ymddiddan Arthur a Glewlwyd Gafælfawr (What Man is The Gatekeeper/Porter?, or Arthur and The Gatekeeper/Porter, or Dialogue of Arthur and Glewlwyd Gafælfawr),
or A Dialogue between Arthur and Glewlwyd/Glewleyd Gavælvawr/Mighty(-)Grasp, otherwise known as Dialogue with Glewlwyd from Llyfr Du Cærfyrddin (Black Book of Cærmarthen). Tenth/Eleventh/Twelfth Century AD (c AD 1100):

‘Arthur and the Porter (The Black Book of Carmarthen XXXI) from The Four Ancient Books of Wales’ at Academy for Ancient Texts. Ancient Texts Library.
http://www.ancienttexts.org/library/celtic/ctexts/bbc31.html

‘Pa Gwr (Llyfr Du Caerfyrddin XXXI)’ at Academy for Ancient Texts. Ancient Texts Library.
http://www.ancienttexts.org/library/celtic/ctexts/bbc31w.html