Slaves. Victorious generals were given control over the sale of war slaves. Although they could give the proceeds or the slaves themselves to their troops (a good way of keeping your personal army personal), they also amassed significant wealth from this.

Found a citation for this, but it's something we discussed in my late Roman Republic seminar in undergrad. See, Slave Systems of Greek and Roman Antiquity, 70, not the newest text, but it gets the job done.

Caesar sold a significant amount of Gauls into slavery. For example in de Bello Gallico 3.16, Caesar executes the leaders of the Venati and sells the rest into slavery :(. The source can be found here: http://classics.mit.edu/Caesar/gallic.3.3.html.

Edit: I realize Caesar's decision to sell the Venati into slavery wasn't purely out of greed, but to instill fear amongst the Gauls, but he also made a pretty penny doing so.

/u/Enrico_Dandolo correctly mentioned slaves--this was surely the bulk of Caesar's wealth. But slaves were far from thr only means for a promagistrate to make money on the provinces. Much of the allure of the praetorship and consulship was the fact that those two magistracies were due promagisterial commands in a province after their year of office. Provincial governors could make enormous amounts of money from all kinds of things. Largely this was embezzlement of various kinds. Governors were given an army, whose equipment (at least the first four legions') was supplied in part by the state. Cicero accused Piso of fabricating cohorts in his account books that didn't exist so that he could pocket the money provided for their equipment. While most taxation was handled by the publicans (with whom governors often made financial deals), not the governor, the governor could exact a grain tax from cities within the province, which was supposed to be used to feed his troops. Piso was also accused of raising vastly more grain than he needed, and then selling the excess for huge sums of money--this appears to have been a very common practice, particularly in grain-rich provinces like Sicily. Further, thr governor's total legal authority and his army could allow him to plunder to his heart's content if he wished. Verres confiscated the property even of Roman citizens, and Piso was accused of attacking and plundering his allies and Greek free cities. Speaking of free cities, such cities, common in Greece, often voted honors and gifts on governors--these were essentially tribute in all but name, but the governor had no legal need to send any of it back to the treasury. Not that governors did so with tribute very often anyway. Many peoples within provinces, particularly frontier provinces like Macedonia or Cisalpine Gaul, were not technically under the subject of the province but were tributaries, and paid tribute to the governor, who could pretty much pocket it if he wanted, no questions asked. Pompey drew gigantic sums of money from his eastern provinces in tribute before Caesar passed his Eastern Settlement in 59, and even then much of the money coming from those provinces went straight to his pocket, since many of those states were his personal clients. It was this plunder and tribute that Crassus so desperately hoped for when he was appointed governor of Syria after his second consulship, in order to equalize his wealth with Pompey's.

Some governors were quite unscrupulous, and others were not. Cicero fell into the latter category, and ironically so probably did Piso, despite Cicero's slander. Verres did not, but Caesar appears to have been pretty honest in his administrative dealings. Still, between slaves and plunder Caesar made a pretty penny, more than he could had he stuck go the traditional embezzlement. A nice thing about a province like Transalpine Gaul was that most of the region was unconquered--by finding excuses to make war in the province, Caesar gave himself the opportunity to secure slaves and plunder. Gaul may have been politically divided, but it was rich--its soil was good, the Gauls had large stocks of precious metals (which they were masters at working), and the province was full of expensive commodities like livestock. To say nothing of the people who could be taken captive. And Gaul was unplundered--between slaves, plunder, and tribute Caesar was set