rule of grammar vs grammatical rule

rule of grammar and grammatical rule both are nouns.

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

As nouns, rule of grammar and grammatical rule are synonyms defined as:

rule of grammar and grammatical rule: a linguistic rule for the syntax of grammatical utterances

rule of grammar (noun)	grammatical rule (noun)
a linguistic rule for the syntax of grammatical utterances	a linguistic rule for the syntax of grammatical utterances