遽然 is read as きょぜん (kyozen) and means sudden.
きょぜん
kyozen
しぜん
nature
とうぜん
naturally
ふしぜん
unnatural
てんねん
natural airhead
いぜん
still
ぜんぜん
(not) at all
こうぜん
open
ひつぜん
inevitability
とつぜん
abrupt
だんぜん
definitely (wrong, different, etc.)
みぜん
before it happens
せきぜん
forlornness