The Collaborative International Dictionary of English v.0.48:

Gregorian \Gre*go"ri*an\, a. [NL. Gregorianus, fr. Gregorius
   Gregory, Gr. ?: cf. F. gr['e]gorien.]
   Pertaining to, or originated by, some person named Gregory,
   especially one of the popes of that name.
   [1913 Webster]

   Gregorian calendar, the calendar as reformed by Pope
      Gregory XIII. in 1582, including the method of adjusting
      the leap years so as to harmonize the civil year with the
      solar, and also the regulation of the time of Easter and
      the movable feasts by means of epochs. See Gregorian
      year (below).

   Gregorian chant (Mus.), plain song, or canto fermo, a kind
      of unisonous music, according to the eight celebrated
      church modes, as arranged and prescribed by Pope Gregory
      I. (called "the Great") in the 6th century.

   Gregorian modes, the musical scales ordained by Pope
      Gregory the Great, and named after the ancient Greek
      scales, as Dorian, Lydian, etc.

   Gregorian telescope (Opt.), a form of reflecting telescope,
      named from Prof. James Gregory, of Edinburgh, who
      perfected it in 1663. A small concave mirror in the axis
      of this telescope, having its focus coincident with that
      of the large reflector, transmits the light received from
      the latter back through a hole in its center to the
      eyepiece placed behind it.

   Gregorian year, the year as now reckoned according to the
      Gregorian calendar. Thus, every year, of the current
      reckoning, which is divisible by 4, except those divisible
      by 100 and not by 400, has 366 days; all other years have
      365 days. See Bissextile, and Note under Style, n., 7.
      [1913 Webster]

WordNet (r) 3.0 (2006):

Gregorian
    adj 1: of or relating to Pope Gregory I or to the plainsong
           chants of the Roman Catholic Church
    2: of or relating to Pope Gregory XIII or the calendar he
       introduced in 1582