Gnaeus Pompeius Maximus, known to modern scholars as Pompey or Pompey the Great, is best known in history for his failure in his civil war with Gaius Julius Caesar. Although Pompey was a competant general and political leader, Ceasar's victory spelled his death and the end of the Roman Republic.
This timeline is based around the idea that Pompey manages to secure victory over Caesar. Caesar's initial victory in Italy was successful mainly because his opponants overestimated him. Caesar, despite being outnumbered two to one, had such a fearsome reputation that Pompey abandoned Rome. This same reputation led the majority of the Plebian Roman Citizens to support him. If Caesar were to lose this reputation, and Pompey were to meet and defeat him on the field of battle, then Rome would not see an emperor for some time, and in the meantime, suffer under the declining Republic.
Caesar's reputation for invicibility was won in Gaul. Of his enemies in Gaul, the most seriuos was Vercingetorix, King of the Arverni and nominal High King of Gaul. Vercingetorix's revolt in this timeline was far more successful, resulting in Caesar being cast from Gaul and the Roman Republic losing the territory that it had, under his leadership, gained. Caesar might have retaken Gaul, if not for Pompey's interferance and the begining of the civil war.