The Collaborative International Dictionary of English v.0.48:

Grammatical \Gram*mat"ic*al\, a. [L. grammaticus, grammaticalis;
   Gr. ? skilled in grammar, knowing one's letters, from ? a
   letter: cf. F. grammatical. See Grammar.]
   1. Of or pertaining to grammar; of the nature of grammar; as,
      a grammatical rule.
      [1913 Webster]

   2. According to the rules of grammar; grammatically correct;
      as, the sentence is not grammatical; the construction is
      not grammatical. --Gram*mat"ic*al*ly, adv. --
      Gram*mat"ic*al*ness, n.
      [1913 Webster]

WordNet (r) 3.0 (2006):

grammatically
    adv 1: in a grammatical manner; "this child already speaks
           grammatically" [ant: ungrammatically]