main vs gas main

main and gas main both are nouns.

Word	Noun	Adjective	Verb	Adverb
main	Yes	Yes	No	No
gas main	Yes	No	No	No

As nouns, gas main is a hyponym of main; that is, gas main is a word with a more specific, narrower meaning than main:

main: a principal pipe in a system that distributes water or gas or electricity or that collects sewage
gas main: a main that distributes gas

Other hyponyms of main include electric main, sewer line, sewer main, water main.

main (noun)	gas main (noun)
a principal pipe in a system that distributes water or gas or electricity or that collects sewage	a main that distributes gas
any very large body of (salt) water

main (adjective)	gas main (adjective)
(of a clause) capable of standing syntactically alone as a complete sentence
most important element
of force; of the greatest possible intensity