Note: Fictional details from the Myst franchise follow, and will be treated as facts.

Gehn talking to the Stranger in the Riven game

Gehn (D'ni: Gehn/[Gen]) was the father of Atrus and the husband of Leira (Keta). As one of the last D'ni people, Gehn was born in the City before the downfall, as the son of the D'ni noble Aitrus and the human Ti'ana (Anna).

Gehn spent a great deal of his life trying to rebuild the fallen empire. He considered himself semi-divine due to his ability to link to Ages he thought he created — a change from the normal D'ni view, which held that the D'ni only linked to existing Ages rather than creating them. Gehn was impatient and did not fully understand the Art of Writing, and as such his flawed links always fell to decay and ruin over time, unlike those of the earlier D'ni or those of his son Atrus, which flourished. This lack of ability to write stable Ages caused great anger in Gehn, and he blamed everything and everyone but himself - not seldom ready to murder his own son for it. To calm his nerves, Gehn smoked a pipe with frog extract in it.

Gehn never fully understood the D'ni, and as such he made many false base assumptions: for example, he mistakenly overrated the significance which the number five held in the D'ni culture, trying to connect items to it, and considering those which are unrelated to be inferior.

Gehn's goals of restoring the D'ni culture were considered honourable, however his methods were considered horribly cruel. He had no regard for the inhabitants of the Ages he linked to and eventually destroyed, believing the inhabitants to be his creations, and therefore not real people. He destroyed and plundered many Ages in order to gain knowledge and resources for restoring the D'ni empire and its people. Gehn's twisted world view likely arose due to the loss of his father at the D'ni downfall: Gehn was already alienated from his non-D'ni mother, and blamed her for the fall of D'ni. Soon after reaching the surface Gehn left Ti'ana, and found a wife in a nearby tribe. When his wife grew ill when near childbirth Gehn finally returned to his mother, but too late: Leira died after giving birth to her son, and Gehn left without even naming the child.

Gehn returned years later when Atrus, as named by Ti'ana, was already in his teenage years. He took his son with him to K'veer in D'ni, and tried indoctrinating Atrus, intent on making him his pupil and servant. However, Atrus recognized his father to be twisted, and rebelled against him: an act which almost caused Gehn to murder his son. Trapped in a room in K'veer with only a link to one of Gehn's Ages as an escape, Atrus eventually Linked through, ending up on Riven, which Gehn only refers to as Fifth Age, since it was Gehn's fifth attempt at a Link to an Age. There, Atrus met Katran (Catherine), a young Rivenese woman. Once Gehn found out, he decided to marry her - without her consent - within 30 days as to gain control over her, and to intimidate Atrus. Working together to stop Gehn's malice, Atrus and Catherine succeeded in trapping Gehn on Riven by removing all linking books from the island, so that there could be no escape for him. In a final showdown, Catherine linked to Myst, an Age she had written in secret together with Ti'ana, and Atrus threw himself into a strange void-like anomaly in Riven called "The Star Fissure." He linked to Myst while falling, but leaving his book to fall through the darkness into the hands of the Stranger leading to the events covered in the game of Myst.

However, Atrus's choice of punishment was hard on the people of Riven, and many had suffered under Gehn's regime. And to complicate matters further, an investigation by Catherine when she returned to Riven revealed that he was not truly imprisoned at all, merely hindered. He made books out of the forests and used the Rivenese as workers to assemble paper and power his books. Catherine was later imprisoned herself by Gehn. Thus, in the Riven game, it is the player's task in Riven to free her and trap Gehn in a more safely manner.