Depends of what you call "Gauls"
Celtic Gauls with the littlest roman influence : Make Ambiorix succed to defeat the Romans army with the help of Germans. Killing Cesar is a bonus.
Ambiorix was the Che Guevara of the Gaul : never captured in battle (in fact, at the difference of Che, never captured at all), seen as a hero by his men and as a bastard by his foes (contrary to Vercingetorix that he praise, Cesar always hated Ambiorix that he never suceed to really defeat).
The Southern Gaul is quite un-receltizable : Pilnus consider the province as another Italy by the culture and the prosperity.
Celtic Gaul with a certain roman influence : Vercingetorix's victory. Vercingetorix didn't much serached to crush the Romans to make them quit the Gaul. No decisive battle of any kind.
A victory of Vercingetorix would certainly allow Caesar or the Romans if the Iulius is dead, to preserve many alliances in Gaul and maybe some territories in Aquitaine and Helvetia.
Northern and Southern Gallo-Roman (the northern gave the french, the southern the occitan) : Succesfull "Gallic Empire". You need the crush of the Roman Empire and no Aurelianus, but it could make it.
Be careful, the so-called "Gallic Empire" ,ever considered itself as "Gaul" and even less "celtic". It was a Roman Empire, with senate, coins, using latin, etc.
The better equivalent would be the Byzantine Empire, that considered itself Roman during all its history.
Northern Gallo-Roman : Defeat of Clovis at Soissons, Syagrius control Northern Gaul from Loire to Rhine.
