income vs financial gain

income and financial gain both are nouns.

Word	Noun	Adjective	Verb	Adverb
income	Yes	No	No	No
financial gain	Yes	No	No	No

As nouns, financial gain is a hypernym of income; that is, financial gain is a word with a broader meaning than income:

income: the financial gain (earned or unearned) accruing over a given period of time
financial gain: the amount of monetary gain

income (noun)	financial gain (noun)
the financial gain (earned or unearned) accruing over a given period of time	the amount of monetary gain