unwelcome guest vs gatecrasher

unwelcome guest and gatecrasher both are nouns.

Word	Noun	Adjective	Verb	Adverb
unwelcome guest	Yes	No	No	No
gatecrasher	Yes	No	No	No

As nouns, unwelcome guest and gatecrasher are synonyms defined as:

unwelcome guest and gatecrasher: someone who gets in (to a party) without an invitation or without paying

Other synonyms of unwelcome guest include crasher.

unwelcome guest (noun)	gatecrasher (noun)
someone who gets in (to a party) without an invitation or without paying	someone who gets in (to a party) without an invitation or without paying