Gabinius was a military tribune under Lucius Cornelius Sulla (Sulla, Lucius Cornelius) and was later sent as Sulla's envoy to Mithradates VI Eupator, the king of Pontus. As tribune of the plebs in 67 he worked to help Pompey solve Rome's major foreign policy problems: the infestations of pirates around the Mediterranean and the long war with Mithradates. He may have delayed Mithradates' defeat by transferring the command of the province of Bithynia-Pontus from Lucius Licinius Lucullus (Lucullus, Lucius Licinius) to the consul Manius Acilius Glabrio, along with part of Lucullus's army. He then set up a command with wide powers intended to be used by Pompey against the pirates. The province was the entire coast of the Mediterranean and all the land within 50 miles (80 km) of the coast. (When his fellow tribune Trebellius vetoed the motion, Gabinius initiated a process of deposition against him, in imitation of a similar motion carried out by Tiberius Gracchus (Gracchus, Tiberius Sempronius) in 133.)

Pompey went on to eliminate the pirates, and he defeated Mithradates, who committed suicide. He then organized most of the Middle East in Rome's interest and his own, with Gabinius serving under him. When Pompey worked with Julius Caesar (Caesar, Julius) and Marcus Crassus (Crassus, Marcus Licinius) to control the Roman state in a secret combination (an unofficial triumvirate), they secured the election of Gabinius as consul for 58 with Lucius Calpurnius Piso Caesoninus. They worked with Publius Clodius Pulcher (Clodius Pulcher, Publius) to undermine the authority of the Senate by driving Cicero (Cicero, Marcus Tullius) into exile. Gabinius was rewarded with the proconsulship of Syria, which he governed from 57 to 54. In Syria he tried to stop the depredations of the Roman knights—who were in charge of collecting taxes—by taking over the tax collecting himself, and profiting thereby. He intervened in Judaea to the advantage of Antipater by restoring John Hyrcanus II as high priest; he also rebuilt towns and introduced administrative reforms.

Under Pompey's instructions, but without the consent of the Senate, he restored Ptolemy XII Auletes to the throne in Egypt—in return, it is said, for a payment of 10,000 talents. He left Roman troops to protect Ptolemy and then had to restore order in Judaea when revolts broke out there. The knights were hostile to him because of his actions in Syria, and he had made other influential enemies as well. An attempt to convict him of treason was unsuccessful, but he was finally convicted of extortion and went into exile in 54, although Pompey had compelled his enemy Cicero to speak on his behalf. He was recalled from exile by Julius Caesar in 49 and fought for him in Illyria (48–47); he died of illness at Salonae, near present-day Split.