punishing vs gruelling

punishing and gruelling both are adjectives.

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

As adjectives, punishing and gruelling are synonyms defined as:

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

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

punishing (adjective)	gruelling (adjective)
resulting in punishment	characterized by effort to the point of exhaustion; especially physical effort
characterized by effort to the point of exhaustion; especially physical effort