Is "gen" also a reading of this kanji? 間

According to a page, this word is pronounced "ningen"

but according to another page, the readings for 間 are

Are they the same kanji? Is "gen" an unofficial reading or something?

Answer

It's called Rendaku. Regularly in compound words, though not always, the second part of a compound ends up voiced. So the reading of the kanji is in essence ケン, but with a dakuten (゛) or voice mark added to it. This gives us the reading ゲン as in the word 人間. It's not a very common reading though, personally I can only think of one other word using the ケン reading, and that is the word 世{せ}間{けん}.

Take the two words fire = 火{ひ} and flower = 花{はな}. Together they can form two new words, fireworks = 花{はな}火{び} and spark = 火{ひ}花{ばな}. The reading is the same in both except that when it appears last in a compound it gets voiced.

Why does the last part get voiced sometimes? I don't know, and I don't think anyone definitely knows. It could make it easier to say, or it makes the two parts more connected into a new words rather than the sum of its parts. Either way it's something the Japanese do and worth knowing about.

Edit: Example, both 忠{ちゅう}告{こく} and 中{ちゅう}国{ごく} have the same on-reading for both kanji, however only the latter one gets voiced.