gruelling vs arduous

gruelling and arduous both are adjectives.

Word	Noun	Adjective	Verb	Adverb
gruelling	No	Yes	No	No
arduous	No	Yes	No	No

As adjectives, gruelling and arduous are synonyms defined as:

gruelling and arduous: characterized by effort to the point of exhaustion; especially physical effort

Other synonyms of gruelling include backbreaking, grueling, hard, heavy, laborious, operose, punishing, toilsome.

gruelling (adjective)	arduous (adjective)
characterized by effort to the point of exhaustion; especially physical effort	difficult to accomplish; demanding considerable mental effort and skill
	characterized by effort to the point of exhaustion; especially physical effort
	taxing to the utmost; testing powers of endurance