In Heinrich von dem Türlin’s Diu Crône, Gawain, in a list of his greatest exploits, mentions that he
took many a drink from the Fountain of Youth
in the Garden of Dochel. This episode is not found in any existing romance.
See also
Fountain of Youth | The Legend of King Arthur
Source
Diu Crône | Heinrich von dem Türlin, c. 1230