In 2368 she was part of an expedition lead by Dr. Marjorie Devarona to investigate recently discovered ruins on the planet Rajatha Prime. Shortly into the team’s time on the planet they discovered a crashed 22nd century shuttlepod. And nearby, in the possession of a corpse, Gest found a collection of valuable harmonic diamonds. Unknown to any of them at the time the diamonds stimulated areas of the brain associated with primitive emotions, including anger, envy, and greed. Their effects began as soon as Gest touched them; she became quite possessive of the gems. However, that night the decision was made to split them between the team, with Jean-Luc Picard opting not to take one.
The next morning Gest contacted her ship to arrange to be collected and then sabotaged the expedition’s comm unit. She killed Dr. Devarona, throwing her into a gorge and then stole her diamond, stashing it in her canteen. When the rest of the team discovered Devarona's body Gest tried to suggest there might be someone else on the planet who committed the deed.
Later that day, back at the ziggurat where the shuttlepod was discovered, Gest killed another member of the expedition, Kob, and then attacked another, Hoyle, killing him with a phaser in full sight of Picard. Then, explaining how Hoyle had killed the rest of the party she agreed to help Picard move the shuttlepod, believing she had him convinced of her innocence.
Behind the shuttlepod the two found a Rajathan ceremonial entrance to the underworld, a pristine chamber of untold value not seen by anyone in two millennia. Acting in awe she attempted to convince Picard she was only interested in the archaeological value of the planet and offered him her harmonic diamond.
Picard called her bluff, well aware, having observed that she had stopped drinking from her canteen since discovering the diamonds, that she had the entire collection in her possession. Infuriated Gest armed herself with a Rajathan blade and backed out of the chamber, confident her ship would be soon be awaiting her.
However, she backed straight into the butt of William T. Riker's phaser. The Enterprise had been alerted to unusual goings on by the lack of scheduled contact with the team due to the comm being disabled. The gems fell into possession of Jean-Luc Picard, who, per Dr. Devarona's final wishes, had them used to fund archaeological digs across the quadrant. (TNG - The Space Between comic: "Captain's Pleasure")