Gavrok was the leader of the criminal organisation known as the Bannermen. He led them in their quest to exterminate the Chimerons.
Biography
Searching for Delta
The Bannermen attacked the Chimeron home planet, Chumeria, intending to wipe the Chimerons out. However two Chimerons escaped in a Bannerman craft. Gavrok lay in wait for them and shot Chima. Before dying, Chima shot at Gavrok, knocking him from the craft. The other Chimeron, Delta, fled in the craft.
Gavrok and the Bannermen followed her to a toll port where they interrogated the tollmaster as to her location. When he proved unable to provide the information, Gavrok killed him.
A mercenary informed Gavrok of Delta's whereabouts: Shangri-La camp, Earth. Gavrok used a high-impulse beam to kill the mercenary so he did not have to pay him.
On Earth
However he had killed him too early. Gavrok did not know precisely where Delta was. He ordered a scan for high technology. They found her and landed, taking Jerome Weismuller and Hawk prisoner. Gavrok ordered two Bannermen to guard the prisoners whilst he and the rest moved on to the camp.
They arrived in time to destroy the Nostalgia Tours bus. Gavrok confronted Melanie Bush, the only survivor, and asked if Delta was aboard. She lied, saying she was. Moments later Delta passed by on a motorcycle. Gavrok decided to use Mel and the camp leader Burton as bait.
The Seventh Doctor approached the Bannermen under a white flag, but Gavrok shot it. The Doctor told Gavrok to let Mel and Burton go; he was already in enough trouble. Gavrok signalled the Bannermen guarding Weismuller and Hawk to place a tracker on the bike and booby-trapped the Doctor's TARDIS with a sonic cone.
Gavrok followed the signal to a dead end. One of the guards told him where Delta's hideout was. Gavrok walked into a trap. The Bannermen and he were covered in honey and Goronwy's bee's attacked them. He reorganised and set out for the camp, where the sound of the Chimeron princess was amplified through a stereo, sending Gavrok and the Bannermen into disarray. Gavrok fell within the perimeter of the sonic cone and was atomised. (TV: Delta and the Bannermen)
Personality
Gavrok wanted to wipe out every Chimeron. He waited aboard a Bannerman craft while his men killed the last Chimerons, predicting that someone would board. He killed Chima whilst his back was turned and taunted Delta when he tried to kill her.
Gavrok told the tollmaster he was free to go and then killed him when he turned away, showing he had little regard for honesty. He had little respect for convention. He fired at the Doctor whilst he was under the flag of truce.
Gavrok was cunning. He booby-trapped the TARDIS in expectation of the Doctor's return. He reacted violently to the incompetence of his troops and often berated them. (TV: Delta and the Bannermen)
Behind the scenes
The script for Delta originally described him as follows: "An awesome sight with red eyes and a black militaristic uniform. Gavrok has a Zap gun slung across his chest and a sling in one hand from which hang the long black pennants of his empire. In the other hand is a curved ram's horn." (INFO: Delta and the Bannermen)