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