arduous vs gruelling

arduous and gruelling both are adjectives.

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

As adjectives, arduous and gruelling are synonyms defined as:

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

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

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