grammar vs grammatical

grammar is a noun but grammatical is not a noun.

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

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

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