The Collaborative International Dictionary of English v.0.48:

Gregory \Gregory\ prop. n.
   1. Pope Gregory XIII., born 1572, died 1585, the pope who
      introduced the modern calendar.

   Syn: Gregory XIII, Ugo Buoncompagni.
        [WordNet 1.5]

   2. Pope Gregory I., born circa 540, died 604; the pope for
      whom Gregorian chants were named.

   Syn: Gregory I, Saint Gregory I, Gregory the Great.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

Gregory
    n 1: (Roman Catholic Church) a church father known for his
         constant fight against perceived heresies; a saint and
         Doctor of the Church (329-391) [syn: Gregory, Gregory
         Nazianzen, Gregory of Nazianzen, St. Gregory of
         Nazianzen]
    2: Italian pope from 1831 to 1846; conservative in politics and
       theology; worked to propagate Catholicism in England and the
       United States (1765-1846) [syn: Gregory, Gregory XVI,
       Bartolomeo Alberto Capillari]
    3: the pope who sponsored the introduction of the modern
       calendar (1572-1585) [syn: Gregory, Gregory XIII, Ugo
       Buoncompagni]
    4: the Italian pope from 1406 to 1415 who worked to end the
       Great Schism and who retired to make it possible (1327-1417)
       [syn: Gregory, Gregory XII, Angelo Correr]
    5: the Italian pope who fought to establish the supremacy of the
       pope over the Roman Catholic Church and the supremacy of the
       church over the state (1020-1085) [syn: Gregory, Gregory
       VII, Hildebrand]
    6: (Roman Catholic Church) an Italian pope distinguished for his
       spiritual and temporal leadership; a saint and Doctor of the
       Church (540?-604) [syn: Gregory, Gregory I, Saint
       Gregory I, St. Gregory I, Gregory the Great]