In the text below, what does "gathering darkness" mean?

The sun has passed from the lake, and as we sit in the gathering darkness voices fall silent and the evening chill descends on us. My grandson has clambered back up into my chair and lies sleeping against my chest; I have wrapped my arms about him against the cold, and we rest in a bonded warmth in the darkening night air.

From Corpus.

May I say "involvig darkness" or "approaching darkness" instead of "
gathering darkness"?