From Middle Dutch guul, gûle, from Proto-West Germanic *gūl- (stem form uncertain), of uncertain origin but possibly related to *geutan (“to pour”). The original sense could have been "powerful horse, male animal," in which case the "pouring" would refer to insemination. Similar sense development is possibly found in Proto-Germanic *ūraz (“aurochs”).
Cognate with German Gaul (“horse”, chiefly pejorative), Middle Low German gûl(e) (“horse, stallion”), whence regional Swedish gula (“old horse”).