How badly does he lose? If the relief force manages to force through and he as to withdraw his troops, he will likely recover and come back. The loss might be a political embarrassment, but Caesar had lost troops before, and it wasn't like this would be a shock. If his army is cut to shreds, he will have to abandon his conquest and his Roman rivals will likely rationalise it by pointing out that it was an illegal war from the start. He's not going to have much of a future in Rome.
In the long run, Gaul is still likely to end up Roman, but it'll take until the next big man needs glory (or Vercingetorix gets stupid).
Down the line, of course, the butterflies can be massive. For one thing, it won't be Caesar trying to establish himself as the Republic's first single ruler. I don't think pompey had the imagination for this kind of thing, but sooner or later it's bound to happen. If we still get an Alt-Principate, its traditions would end up looking quite different.