gymnosperm vs seed fern

gymnosperm and seed fern both are nouns.

Word	Noun	Adjective	Verb	Adverb
gymnosperm	Yes	No	No	No
seed fern	Yes	No	No	No

As nouns, seed fern is a hyponym of gymnosperm; that is, seed fern is a word with a more specific, narrower meaning than gymnosperm:

gymnosperm: plants of the class Gymnospermae having seeds not enclosed in an ovary
seed fern: an extinct seed-producing fernlike plant of the order Cycadofilicales (or group Pteridospermae)

Other hyponyms of gymnosperm include progymnosperm, welwitschia, Welwitschia mirabilis, cycad, pteridosperm.

gymnosperm (noun)	seed fern (noun)
plants of the class Gymnospermae having seeds not enclosed in an ovary	an extinct seed-producing fernlike plant of the order Cycadofilicales (or group Pteridospermae)