The Collaborative International Dictionary of English v.0.48:

Gradational \Gra*da"tion*al\, a.
   By regular steps or gradations; of or pertaining to
   gradation.
   [1913 Webster]

WordNet (r) 3.0 (2006):

gradational
    adj 1: taking place by degrees [syn: gradational, gradatory,
           graduated]