AMBROSIAN CHANT VS. GREGORIAN CHANT
http://www.canticanova.com/articles/misc/art791.htm
Ambrosian chant developed in around A.D. 384. It is derived from St. Ambrose, bishop of Milan, but none can be attributed to him. St. Ambrose did however, write at least ten hymns.
Gregorian chant came about in the mid 8th Century and developed from the Roman and Frankish chant.
Both types of chant are monophonic including a melodic unison line without accompaniment.
Ambrosian chants are not always syllabic, one note per syllable. It is less uniform than the Gregorian and less organized. Gregorian chants have more structure and the chant is chosen from 8 church modes. Ambrosian chant uses the first four of these modes which are labeled Dorian, Phrygian, Lydian and Mixolydian.
Gregorian chant, developed around 590 A.D. and is an integral part of the Roman Catholic liturgy.
This information is taken from combinations of the Catholic Encylopedia, Encylopedia Brittanica and Grove Music Encylopedia. The hyperlink above is from a blog page found on the Cantica Nova website