No such thing as a hidden method of making lots of gil. There are small ways of making gil such as buying cheap from NPC and selling on AH in a different nation. That method is time consuming and extremely boring!!!!
All the rich people in the game are either high level crafters, constant NM campers, or gilbuyers. And that is 100% true. If they quest to make gil or low level craft or farm, they are NOT rich lol.
Your best bet is to find a few friends you trust and camp big NMs with them daily. By big NMs I mean 21-24 hr NMs. I do this all the time because I think crafting is boring. This method is great if you have multiple accounts so if you have 6 people killing, say King Arthro, then you will get 2/6 of the gil that comes from drops.
Also, stop asking for hidden gil methods because there is no such thing. You guys are absolute idiots and noobs to be asking for !@#$%^&* like this because all high-level players know the truth about FFXI and how hard it sucks the big one
Now good luck, use all the tools on this site's confirmed bots, scripts, and exploits to claim the NMs. It is true, most people don't use hacks but the ones that do are good so you will need all the claiming hacks you can get to be able to claim the NM(s).
I have done my part, now I'm expecting loads of stupid noob flamers and others who will agree with me. So let the conflageration begin !! haha
(Oh btw, I will respond to some of the flamers because I know what I said above this is absolutely true and I don't plan to back down from my statements

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And just for the record, if you aren't making BIG BULK of gil in short amt of time, then you aren't fully utilizing your resources. But I guess we need people like you who go out and farm small items so the high-level players have items readily available at the AH for quests or crafting or whatever else.
One more thing I forgot to mention is a tip for all that are in the mid or low levels and want to make gil. Well the truth is that you guys can't make good until you are high level so I suggest you xp as much as possible and get 60+ so you can camp low level NMs easily on your own. But continue to strive for 70-75 so you can move on to harder ground HNMs.