A limiter was a brain implant used by the Federation. It prevented the implantee from intentionally killing.
Gan was fitted with a limiter at some point prior to his being put on board the London. He was still capable of fighting physically and holding a weapon but could not pull the trigger even when his life was threatened. The limiter occasionally gave him headaches and he worried that it might malfunction and cause him to hallucinate.
The limiter was presumably an experimental device still being tested, as it seems unlikely even the Federation could justify invasive brain surgery on someone not yet found guilty. If the limiter was a piece of standard and tested technology there would be little value in placing one inside someone already sentenced to permanent exile on an isolated planet.
Gan's limiter later suffered a partial malfunction resulting in his becoming almost irrationally violent towards his friends and the ship. His personality underwent a drastic change as well and he became uncharacteristically devious. It may have been that as well as blocking the ability to kill, the limiter effected some kind of personality alteration and the devious and violent personality was Gan's natural state.