The Collaborative International Dictionary of English v.0.48:

Gentlemanlike \Gen"tle*man*like`\, Gentlemanly \Gen"tle*man*ly\,
   a.
   Of, pertaining to, resembling, or becoming, a gentleman;
   befitting a man of good breeding; well-behaved; courteous;
   polite; as, gentlemanly behavior.
   [1913 Webster]

WordNet (r) 3.0 (2006):

gentlemanly
    adj 1: befitting a man of good breeding; "gentlemanly behavior"
           [syn: gentlemanlike, gentlemanly]

Moby Thesaurus II by Grady Ward, 1.0:

38 Moby Thesaurus words for "gentlemanly":
   andric, aristocratic, bull, chivalrous, cultivated, cultured,
   ducal, exalted, genteel, gentle, gentlemanlike, high, highbred,
   kinglike, kingly, knightly, ladylike, male, manful, manlike, manly,
   mannish, masculine, noble, of gentle blood, of rank, patrician,
   polished, princelike, princely, queenlike, queenly, quite the lady,
   refined, titled, uneffeminate, well-bred, well-brought-up