Some of this has to do with a bit of geographic confusion, as "Gaul" is not actually the same as "France" in modern terms, though it is often popularly understood as such. What Romans of the time called "Gaul" (as in, lands inhabited by peoples they referred to as "Gauls") was divided into two provinces: Cisalpine Gaul (Gaul on the "near side," or Italian side of the Alps), and Transalpine Gaul (Gaul across the Alps from Italy, what we would call France).

Cisalpine Gaul incorporates much of what is now northern Italy that includes the Po Valley, and is bounded on the south by the Rubicon River.

A bit of background here: when his consulship ended in 59 BCE, Caesar secured the proconsulship for Cisalpine Gaul, which let him keep an eye on the situation in Italy, but crucially, he had Pompey and other political allies extend his proconsulship to cover Transalpine Gaul as well, which would give him the opportunity for conquest and plunder. Over the next decade, Caesar would go on to conquer, pillage, and massacre his way through Gaul, which I'm sure you're familiar with.

His position as proconsul was slated to end in 50 BCE, and Caesar wished to return to Rome for a triumph, and ideally to stand for election as consul again. However, since his alliance with Pompey had dissolved, and he had made a number of powerful enemies in the city, he evidently believed that he would have been prevented from doing so. Whether he actually feared prosecution for actions taken when he was consul, simply feared the loss of his dignitas if he did not achieve his goals, or was just looking for a pretext to march on Rome is a more complex question than I can answer here, but while he was waiting for the situation to resolve, he was camped in Cisalpine Gaul, on the eastern side of the Italian peninsula, hence his immediate access to the Rubicon.

Hope that answers the question. It doesn't help matters that almost immediately after taking control of the city, Caesar pushed through a law that made people in Cisalpine Gaul citizens (something that they had long desired and was key to his political popularity there), and then it was merged into Italy proper in 42 BCE, so later references to "Cisalpine Gaul" are often absent.