gas main vs main

gas main and main both are nouns.

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

As nouns, main is a hypernym of gas main; that is, main is a word with a broader meaning than gas main:

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

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

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