How would Gauls in 50 BC refer to the current year?

The Gallic calendar was a hybrid lunisolar calendar, divided into periods of five and thirty years; but AFAIK there’s no record of how those periods synchronized with known historical epochs.

As for the Romans, they commonly referred to years by whoever served as consuls for the year: 50 BC was the consulate of Lucius Aemilius Lepidus Paullus and Gaius Claudius Marcellus Minor.