The Collaborative International Dictionary of English v.0.48:

Grammatic \Gram*mat"ic\, a.
   Grammatical.
   [1913 Webster]

WordNet (r) 3.0 (2006):

grammatic
    adj 1: of or pertaining to grammar; "the grammatic structure of
           a sentence"; "grammatical rules"; "grammatical gender"
           [syn: grammatical, grammatic]

Moby Thesaurus II by Grady Ward, 1.0:

39 Moby Thesaurus words for "grammatic":
   adjectival, adverbial, attributive, conjunctive, copulative,
   correct, descriptive, formal, functional, glossematic,
   glottochronological, graphemic, intransitive, lexicographic,
   lexicological, lexicostatistical, lingual, linguistic, linking,
   metalinguistic, morphological, morphophonemic, nominal,
   participial, philological, phonemic, phonetic, phonological,
   postpositional, prepositional, pronominal, psycholinguistic,
   semantic, structural, substantive, syntactic, tagmemic, transitive,
   verbal