Caesar was quite a prolific writer, which is perhaps not surprising since he was considered even by Cicero to be among the greatest orators and rhetoricians of his day. We know of the titles of quite a number of lost works. Suetonius mentions the de Analogia, the Anticatones (speeches against Cato), and a poem called the Iter (the Journey). Suetonius also mentions a number of works from his youth, the Laudes Herculis, a tragedy called the Oedipus, and there was a collection of his sayings that Augustus refused to allow to be published. There was also a collection of letters and a collection of speeches, from which Augustus had a couple removed for doubtful authenticity. Tacitus was of the opinion that Brutus' and Caesar's poetry was no better than Cicero's (which was universally panned in antiquity), but that they were luckier because fewer people knew about their work--Tacitus seems to imply that they mainly distributed their poems to the collections of their friends (fecerunt enim et carmina et in bibliothecas rettulerunt, "for they wrote poems as well, and brought them back into libraries"--whether that means their private collections or those of their friends is a bit unclear, but obviously these works didn't have wide circulation, unlike Cicero's). Badian supposed that Caesar must've written in his spare time as a sort of way to kill time and keep himself buys (Suetonius says that the man was eternally busy, and would sleep in his carriage because he would work through the night). This is certainly supportable from our texts. Suetonius says:

quorum librorum primos in transitu Alpium, cum ex citeriore Gallia conventibus peractis ad exercitum rediret, sequentes sub tempus Mundensis proelii fecit; novissimum, dum ab urbe in Hispaniam ulteriorem quarto et vicensimo die pervenit

Of which books [the De Analogia, Anticatones, and the Iter] the first he wrote on his journey through the Alps, when he was returning to his army from Cisalpine Gaul with his assembly finished [the Conference at Luca], the next at the time of the battle of Munda, and the newest while coming to Further Spain from the city in twenty four days

Which certainly seems to support the idea that Caesar rattled these works off in short order, particularly the fact that he wrote the Iter, which must've been a poem of some length, in only 24 days while on a journey! Plutarch supports the idea that these were a sort of sport for Caesar, saying that he composed poems and speeches for the pirates while he was their hostage, joking whenever one of them didn't like it that that guy was a barbarian. Pliny listed Caesar as one of many men who, like himself, dabbled in amusing himself with writings for fun