grammatical vs grammar

grammatical is not a noun while grammar is a noun.

Word	Noun	Adjective	Verb	Adverb
grammatical	No	Yes	No	No
grammar	Yes	No	No	No

grammatical (noun)	grammar (noun)
	the branch of linguistics that deals with syntax and morphology (and sometimes also deals with semantics)

grammatical (adjective)	grammar (adjective)
conforming to the rules of grammar or usage accepted by native speakers
of or pertaining to grammar