My understanding is that corn, or maize, existed solely on the American continents up until the 15th century and the arrival of European explorers. But Caesar refers constantly to securing a supply of "corn" for his troops. Does the word have another meaning in this context, perhaps just a generic term for food? What food is he referring to? What would his army have been eating as they moved around Gaul throughout the seasons?
Caesar is likely using the Latin word "frumento" or perhaps its plural, "frumenti," to refer to wheat or another grain crop. If you're reading a British translation of Caesar, they're most likely using "corn" in the British sense for a staple crop or grain crop, often wheat but not always. Using "corn" to refer to maize is a bit of an Americanism; you'll hear maize called "Indian corn" in Britain.
The British Corn Laws (tariffs on imported grain) in the mid-19th century always puzzle American students.