ground level vs first floor

ground level and first floor both are nouns.

Word	Noun	Adjective	Verb	Adverb
ground level	Yes	No	No	No
first floor	Yes	No	No	No

As nouns, ground level and first floor are synonyms defined as:

ground level and first floor: the floor of a building that is at or nearest to the level of the ground around the building

Other synonyms of ground level include ground floor.

ground level (noun)	first floor (noun)
the floor of a building that is at or nearest to the level of the ground around the building	the floor of a building that is at or nearest to the level of the ground around the building
the height of the ground on which something stands