A lot of people will tell you to level your crafters, your gatherers, and your retainers. Most won't tell you the enormous amounts of gil you will pour into them doing so. IOW, if you need gil, leveling all these classes isn't going to do that for you. However, if you pace your leveling instead powering through with gil, you can level all of these classes and even turn a profit.

The thing is, a big reason to level all of this things is to become self-reliant. You will find that by the time you level all of these classes and get them geared, you don't really need gil as you have the means to acquire whatever it is your after yourself. Becoming a millionaire is kind of thing that just naturally happens from having so many ways to supply a demand.

Last, making tons of gil in a very short time can happen for non-crafters/gatherers as well. For example a lucky EX primal run can grant your items that crafters will pay generously for. Done with DF, toss a /shout to see if anyone wants the item, and done. In the end though, the way you make gil in this game is by making more than you spend. You simply have to find the method that works for you, and be ready to find more of them based on what you're doing at the time.