If you're wondering about logistics in general, this question has already been asked a few times before: https://www.reddit.com/r/AskHistorians/comments/1ti44i/how_did_they_deal_with_the_logisticssupply_chain/?st=J5PGFVLY&sh=c51eccec
First, I'll talk about how would you get there and back. Researchers at Stanford recently completed the ORBIS project, which is a Geospatial Network Model of the Roman world:
It is basically a Google Maps for the late Roman world. It factors in known roadways, shipping traffic, etc. It is amazingly detailed, and takes into account details such as roads used, whether the traffic is civilian or military, type of traffic (foot, oxcart, riders, etc), month of the year, and more. It can even calculate, in denarii, the per pound cost of cargo and the price per passenger if traveling by carriage.
According to ORBIS, a military courier on horseback could travel from Rome to Lugdunum in 12 days (I choose Lugdunum due to location - a formal Roman city did not yet exist there, only a small settlement of Gauls). If you used horse relays, a message could be delivered in 5 days, all on perfectly serviceable Roman roadways. This is not a cheap trip (730 denarii to go in a carriage), but not especially perilous either.
An organized campaigning army will maintain guarded and patrolled supply lines back to a population center, where the logistical arrangements are being made to feed the army. Simply put, a large army does not rely on foraging as a primary way to feed itself unless it is cut off and desperate. The Romans were more organized in terms of supply lines than early medieval armies (my field of study), but even these later, less organized armies were largely tethered to whatever local population center was currently under their control.
I am not sure if Caesar's army was fed with food from Lugdunum and its environs, but I am positive that food being sent to the army would have to at least use that roadway to enter Gaul. There are only two roads out of Lugdunum so that part would be easy, and from there it's a matter of leapfrogging small settlements and outposts, while getting latest news and directions from the small garrisons and passing patrols. There's a chance you would enlist/enslave a local guide just to ensure you don't get off track. Most places of note in Gaul would have been a week's ride or less from Lugdunum. This part would be a bit more risky and prone to violence as these roads were lower quality and less traveled, but you'd still be encountering a fair amount of Roman traffic.
When you arrive at whatever outpost is the final stop, you'd either find Caesar, or a small contingent would escort you to the army if it was in the field. They would then escort you back to the outpost, and you follow the road home.
Basically, what I want to communicate here is that knowing how to navigate the Roman world was not that challenging, and it had well established and well maintained transit infrastructure right up to doorstep of Gaul. The important point here is that there would be almost no chance that you would ever find yourself lost on this journey.
u/x--BANKS--x's response is fair for a general sense of communicating long distance in the Roman empire but we actually know more of the specifics about Caesar's campaigns and the way he stayed in touch with Rome.
First off, Caesar spent his winters in a town in northern Italy called Luca. At the time, this city was not considered a part of Italy proper but a province called Cisalpine Gaul and thus in Caesar's scope of command. While there he often entertained the leaders of the city including men like Pompey and Crassus. If you had private business to discuss with Caesar, this is how you did it. So if you wanted to send a message to Caesar you could wait until he was expected at Luca.
Beyond that, most messengers sent to Caesar would have been sent from his own household. When Roman politicians went abroad (or even left the city for the countryside) their homes in the city became a clearing house for their mail. We know this from letters of Cicero where he praises his wife for keeping the mail coming through while he is away (Ad Familiam 14.1-4). Caesar would have had the same system set up. The people in his household would collect mail and once there was enough to send off, a messenger would leave for Gaul. His household would likely also handle mail going to any of his officers. Another similar option was to send mail through the public contractors responsible for collecting taxes in the empire (publicani). They would have a lot of connections throughout the provinces so you could go to the firm/person in charge of Gaul's taxes (or the guy who is trying to buy up all the enslaved Gauls to sell in Rome) and they could forward your message through their network to the province. It would be an extraordinary expense to send a slave/freedman of your own all the way to Gaul not just in terms of the cost of travel but also in the loss of manpower he would have provided otherwise.
The final issue that ORBIS does not really highlight is that many messages were sent in a chain (sort of like modern mail). A courier from Rome might hand the mail off to another courier at Luca who might go as far as Arles and so on. Romans did also have a distinction between private couriers and military couriers, so the courier from Rome could hand the message off to a military courier at one of Caesar's camps. This system was less reliable than having one courier but it was far less work and time for each courier along the way and thus was pretty typical.
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