taru-adjective, adverb taking the 'to' particle
Literary and somewhat archaic; describes a dignified, imposing, or stern manner. Often used adverbially with と (げんこと) or as a たる adjective in formal writing.
彼はげんことした態度で会議に臨んだ。
He attended the meeting with a grave demeanor.
裁判官はげんこと判決を読み上げた。
The judge solemnly read out the verdict.