Eleanor Dickey, a professor of Classics, responded recently to a question about the works read by those learning Latin as a second language in the Greek-speaking ancient world:
[Students in the East] start on Virgil (mainly the Aeneid) and Cicero (mainly the Catilinarians) and then read Terence, Sallust, Seneca’s tragedies, and Juvenal. Caesar is conspicuous by his absence. [emphasis added]
That last sentence is intriguing, since Caesar's works play such an important role in modern Latin instruction.
When did the works of Caesar, like the Commentarii de Bello Gallico, begin to be used widely for Latin language instruction among non-native speakers?