Good point. The French were fighting a two front war.As for your question, Caesar conquered Gaul because Gaul was being invaded by Ariovistus, a Germanic Suevi, at the same time, and marauding Cimbrians and Teutons had also devastated much of southern Gaul, allowing the Romans to pour in from Italy without much resistance. Caesar invaded Gaul using precisely the excuse that Ariovistus was invading and the Aedui, a Celtic people allied with the Romans, requested their aid.
This is not a reason for why C conquered G in such a short time period but is a consequence of his conquering Gaul.Yet another reason, aside from Germanic rampages, is that Caesar straight up committed genocide. He killed about 1 million Gauls and enslaved a million more.
Good point, though crossing Alps did present difficulties.The French since Philip Augustus developed a standing navy or something close to it, and even after the Battle of Crecy where the English gained permanent territories in France, they still had to transport their troops by water and still did not control the whole French coast. It was therefore very difficult to keep control of France. The Romans didn't have this problem.
I've already discussed this above. The Romans did have more people but they had to use those people for defending others parts of the Empire.We have to add that France was far more populated than the whole of Britain, which scarcely numbered more than 5 million if that during the whole of the Middle Ages while the French by the Hundred Years War were almost triple that number. Gaul, meanwhile, was about as populated as Italy, being in the 5-6 million range while Italy was in the 6-7 million range, and that's not the entire empire of the Roman Republic, which at that point was approximating the 15 million range.