old man vs gaffer

old man and gaffer both are nouns.

Word	Noun	Adjective	Verb	Adverb
old man	Yes	No	No	No
gaffer	Yes	No	No	No

As nouns, gaffer is a hyponym of old man; that is, gaffer is a word with a more specific, narrower meaning than old man:

Other hyponyms of old man include codger, old codger, antique, old-timer, old geezer, oldtimer, patriarch.

old man (noun)	gaffer (noun)
an informal term for your father	a person who exercises control over workers
a man who is very old	an elderly man
a familiar term of address for a man	an electrician responsible for lighting on a movie or tv set
(slang) boss
aromatic herb of temperate Eurasia and North Africa having a bitter taste used in making the liqueur absinthe