As for how all the gil was generated, that was answered on reddit. If you look at the numbers, there are only 1M FF14 players. Playing through and questing gets you probably about 250-300K. If every player had done that process, that'd be 300K * 1M, i.e. still short of the amount banned. That doesn't even include the fact that obviously not everyone has done so. The answer given on the FF14 subreddit was that there was a gil generation exploit discovered which turned wet bombard ash into sparklers which were then sold for 10gil each. Speedhacks were employed and it resulted in the generation of (allegedly) millions of gil an hour. Additionally, supposedly there was/is another exploit which has not been explained as of yet.