The Collaborative International Dictionary of English v.0.48:

Glossator \Glos*sa"tor\, n. [LL. See 3d Gloss.]
   A writer of glosses or comments; a commentator. [R.] "The . .
   . glossators of Aristotle." --Milman.
   [1913 Webster]

Bouvier's Law Dictionary, Revised 6th Ed (1856):

GLOSSATOR. A commentator or annotator of the Roman law. One of the authors
of the Gloss.