An Etymological Dictionary of the German Language/Annotated/Gaul
Gaul, masculine, ‘steed, nag,’ from Middle High German gûl, masculine, ‘boar, male animal (generally)’; only at a late period and rarely ‘nag,’ which meaning becomes prominent in the 15th century; for a ‘sorry jade’ runzît is used in Middle High German; Dutch guil, feminine, ‘a mare that does not yet bear.’ The word is not known to the other dialects; its origin is obscure.