step by step vs stepwise

step by step is not an adjective while stepwise is an adjective.

Word	Noun	Adjective	Verb	Adverb
step by step	No	No	No	Yes
stepwise	No	Yes	No	Yes

As adverbs, step by step and stepwise are synonyms defined as:

step by step (adjective)	stepwise (adjective)
	one thing at a time

step by step (adverb)	stepwise (adverb)
in a gradual manner	proceeding in steps
proceeding in steps