foregone vs bygone

foregone is not a noun while bygone is a noun.

Word	Noun	Adjective	Verb	Adverb
foregone	No	Yes	No	No
bygone	Yes	Yes	No	No

As adjectives, foregone and bygone are synonyms defined as:

Other synonyms of foregone include bypast, departed, gone.

foregone (noun)	bygone (noun)
	past events to be put aside

foregone (adjective)	bygone (adjective)
well in the past; former	well in the past; former