Let's use the Gallic Wars as an example. Caesar writes it, finishes it and then... what?, does he pay someone to make copies?, how many copies?, who buys them?, who even reads the book?, could the average roman hear about Caesar's book and decide to check it out?.
You will likely find this instructive
I should point out that the example you mention is probably the single worst example for approaching the question of the publication of classical texts. The question of the publication of the de bello Gallico is fraught with so many problems regarding the purpose, date of publication, and even identity of the work--we're not even sure what exactly the de bello Gallico is supposed to be--that it is not a useful starting point to approach the issue of ancient textual publication. This, on top of the general frustrations with trying to sort out ancient publication.