He preferred to be referred to as "captain" by soldiers, rather than "doctor".
Captain Gentry had no experience fighting against the zerg.
Captain Gentry interrogated one of the survivors of the Siege of Cask on Sorona, Private Maren Ayers, a medic from the 128th Platoon "Iron Jesters". The doctor was particularly interested in the new strain, the baneling, the zerg used to destroy the mining outpost of Cask. However, he had little information on the tactical situation, either not having seen or being unable to properly analyze Cask's natural defenses. He had a much better understanding of the scientific situation, however.
Over the course of the interrogation, Gentry was repeatedly irritated by Private Ayers' inability to stick to the topic, outbursts of emotion, and lack of respect for the Dominion and her superior officer. Gentry was also dismissive of Private Ayers' education, even though she had reached (but not graduated from) graduate school.
As Ayers described how the Dominion had abandoned the colonists of Sorona, Gentry was coldly dismissive. Ayers told how the colonists had been holed up in the naturally well-defended outpost of Cask for eight months, weathering daily zerg attacks, before her platoon arrived. By that point, the zerg numbers had been diminished, and only basic breeds like zerglings had been seen on the battlefield.
After six months of helping the colonists fight the zerg, Private Ayers found herself thinking about her father's theories. Dr. Ayers believed a population's mutation rate increased as the population size decreased. Private Ayers came to believe the zerg were using the situation to increase the mutation rate of their own minions, so she took samples of dead zerglings and tested them in a lab made from parts of her own armor. She found acidic pustules during an autopsy, and when she tried to cut it open with a laser, she accidentally detonated it, causing severe damage. The zerg abruptly stopped attacking, and after a week the Iron Jesters scouted the area. They found large numbers of zerglings mutating into a new breed and killed many of them. Private Ayers realized that the zerg had gotten the mutation they wanted, and were now mass-producing the new minions.
A swarm of banelings fell onto Cask. Private Ayers contacted the Dominion and gave them the new intelligence. The Dominion finally responded, reaching Cask four days later with a fleet of battlecruisers. However, by that point Cask had been destroyed and the Iron Jesters reduced to two people, the commanding officer (Lieutenant Travis Orran) and Private Ayers herself. Ayers complained bitterly about the Dominion's abandonment, prompting Captain Gentry to caution her again.
However, Ayers had had enough. Ayers took Gentry hostage with a smuggled baneling pustule, powerful enough to put a hole in the ship's hull, and accused the Dominion of callously abandoning Cask for research purposes. The doctor called for security, but Ayers was fine with that, since what she had to say would be relatively short. Ayers revealed she had seen spore cannons during the last four days of the siege. The zerg could have used them to destroy Cask at any time, but instead were using them to transmit information to other zerg-inhabited worlds. She warned Gentry that the Dominion was probably unprepared for the return of the zerg. The medic then detonated the baneling pustule before security arrived.