TL;DR: Do your main story, get retainers, and get crafting.

My recommendation for if you want to make gil. Do your main story quest. You need to get to a higher level as well. Also the main story will unlock practically everything you need, (blue quests unlock other things, some you may need, some you can do without.)

You will need your retainers. They're how you sell stuff on the markets. Also, get your crafting and gathering classes going. Crafters are the ones that really make a lot of money in this game. Gathering will get you the supplies you need for crafting.

I'd recommend setting up one of your retainers as a gatherer and your other one as some sort of Disciple of War/Magic. Your gatherer one can gather stuff that you yourself could gather, but it will save you the grind. I usually end up having mine gather all the crystals I need for crafting. Also your Disciple of War/Magic can hunt you some of the stuff you may need for crafting.