I am just going to go ahead and say that the majority of people asking this question are too lazy to actually do anything about it. Level your crafts and gatherer professions. Personally, I've made about 2.6 million gil since early access just selling random stuff I can make with whatever mats I happen to get. Mostly HQ stuff. I have a hard time NOT making something HQ with all NQ mats anymore.

Get out there and farm skins or level LTW yourself and turn them into leather. There are plenty of people that have crafts and are too lazy to farm their own materials. Getting rich on other people's laziness is a way to make gil.

Flip the market. Buy low, sell high. made a few hundred thou doing that. Craft and fill the market holes with your wares.

Nobody will give you a definitive answer about making gil. I will tell you that if you ignore your crafts and gathering, you're doing it wrong. Both are such a joke to level now compared to 1.0...there is no reason not to.