So I'm reading a book about he ten best generals in history and in Ceaser's chapter they mention while he was in Rome they got word about his or another general's army about to have their supply lines cuts so he hustled back to Gaul to support them. My question is how could they get word to Rome and have him make it back to Gaul before the attack happened?
Are you referring to Caesar's campaign against the Helvetii?
At the time of the uprising led by Vercingetorix Caesar was in Gallia Cisalpina (northern Italy), not in Rome. Most of his legions were in their winter camps in Gaul accross the Alps. I'm unsure how your book got the idea that their supply lines were about to be cut; part of being in a winter camp is having enough supplies stored to last a good while.
Anyway Caesar got word of the uprising several weeks after it had started. He was effectively seperated from his main force and decided it was best if he'd move to his forces rather than the other way around (as that might involve his legions doing battle without him).
Caesar took the provincial garrison as well as some fresh recruits, crossed the Alps, and started pillaging the countryside. Under the pretext of going back to Gallia Cisalpina to raise more troops, Caesar left his small army. This drew Vercingetorix and his army towards the Alps.
Meanwhile Caesar joined a small cavalry detachment and made forced march well around Vercingetorix' army. It took several days (according to Caesar's reports, they travelled "day and night" during this time) before Caesar was able to join two of his legions in central Gaul. From there he sent messages to the other winter camps to join him.
To answer your question "How did emergency troop replacements ever make it in time?" in the case of Caesar:
Sources: