get together vs session

get together and session both are nouns.

Word	Noun	Adjective	Verb	Adverb
get together	Yes	No	Yes	No
session	Yes	No	No	No

As nouns, session is a hyponym of get together; that is, session is a word with a more specific, narrower meaning than get together:

get together (noun)	session (noun)
a small informal social gathering	a meeting devoted to a particular activity
	a meeting for execution of a group's functions
	a meeting of spiritualists
	the time during which a school holds classes

get together (verb)	session (verb)
get together socially or for a specific purpose
become part of; become a member of a group or organization
get people together