I know that the main reasons for his campaign were to become even more beloved by the plebs and to repay his debts, but these reasons don't show me why did he prefer Gaul. What were its advantages? Why didn't he choose for example some eastern region which were considered traditionally rich?
We don't know exactly why Gaul was the ultimate result, but there is more nuance. During his consulship in 59 BCE, Caesar's pet tribune Vatinius passed a law of the people securing Cisalpine Gaul and Illyricum for Caesar. Cisalpine Gaul is the part of the province which was on the Italian side of the Alps, roughly equivalent to the NW of the Po valley today.
It is often suggested that Caesar had an eye towards central Europe as the site of a campaign. I, however, believe that the first goal was more simple: to control legions under his imperium which were close to Rome. He got three legions in Vatinius' law, and Cicero was immediately alarmed by this course of events, and the senate in general feared a repeat of the bloody rise of the dictator Sulla only a generation previous. To give the young Caesar, whose consulship was so tumultuous, command of three idle legions in the north of Italia must have seemed terrifying to the senate. (see Cicero Ad Att. II.16; 17; 18, etc).
This might be why they responded by "awarding" Caesar Transalpine Gaul (the area around modern Marseilles and along the coast back to the Alps) and another legion along with it. Perhaps they wanted to lure him (and the legions) further away from Italy and put a big mountain range between the two. We will never know if that strategy was effective, because the actions of the Helvetii, surely unknown to Caesar beforehand, forced Caesar to bring all the legions under his control (and then some) beyond the frontier into Gaul almost immediately upon his taking command in 58. From there, the Gallic Wars unfolded more or less year by year, as more and more of the Gallic contingents were drawn into the conflicts.
There is zero evidence that Caesar had planned an invasion of Gaul (for money or glory or whatever else) before he arrived there in 58, and in fact, from what little information we have, it seems that he had left the original legions which accompanied his command back in Italia and had to raise them quickly when things went sour at Lake Geneva with the Helvetii.