Taking part in Treasure Maps can be a great way to quickly make some Gil, but repairing the full set of gear it rewards costs a lot of Gil. That's a pure overhead cost that you want to avoid as much as possible.
Gathering
If a player is not in the market for new gear or other upgrades, they can still make a good amount of Gil by flipping items on their server’s market board. The key to this strategy is knowing what is selling well and catching the buy/sell trends as they change. This takes some research, but the payoff is high if done right.
Another major source of Gil is the in-game crafting materials. Most crafters are constantly purchasing ingredients, and many of these sell for a decent profit at the market board. This is particularly true for those that specialize in a particular class, such as Carpenters who can rake in Gil with furniture recipes.
For those with a gathering class, there is also a great deal of money to be made by harvesting natural resources throughout Eorzea. This is especially lucrative for those that have a higher level Miner or Botanist skill, as they can gather faster and consume much less GP than their lower-level counterparts.
Crafting
In a game as big as FFXIV, it's important to have a variety of strategies for earning Gil. These can range from the obvious -- AFKing at the Limsa Aetheryte -- to the more obscure, like dye farming. Dye is an extremely popular item and players spend a lot of money on it, which can add up quickly.
It's also wise to sell materials to players that are in need of crafting items. Many overworld mobs drop items that can be sold on the MB for a nice profit. This doesn't even require a gathering Job to do!
Lastly, leves can be a very profitable method for earning Gil. Especially the higher-level roulettes, which give out great materia that will sell for a nice amount of ffxiv gil prices. However, keep in mind that you'll only earn a small amount every day, as leve allowances only recharge at the rate of three per 12 real-world hours. This doesn't really make it worth the effort for most players.
Retainer Ventures
One of the most popular ways to earn Gil in FFXIV is through retainer ventures. Retainers are able to collect certain items that can be sold on the market board for a significant amount of money. However, a player must have the correct level to access this method of earning.
After completing the main Scenario Quest and level 17 in your job, you will be able to send your retainers on so-called "retainer ventures." These are special missions that reward several items and a decent amount of experience.
These adventures can also be used by the gathering classes to harvest valuable loot. Depending on the class and rank, this loot can sell for a significant amount of money. This is one of the most consistent and passive ways to earn Gil in FFXIV.
Selling
The best strategies to make Gil quickly in FFXIV often involve crafting and gathering. Unlike combat-oriented methods that can be time consuming, these are relatively hands-off tasks that can yield consistent results.
Players can rake in a good amount of money by selling their crafted items on the market board. Even a few pieces of the most common crafted items can consistently sell for a profit, as there are always other players in need of their goods.
Gardening can also earn a player quite a bit of money. Certain plants, such as Thavnairian Onions, can yield a substantial profit when sold on the marketplace. Moreover, many optional content rewards such as Skysteel Scrips can be sold for a significant sum. Likewise, dyes that are hard to come by in the game can easily sell for a decent amount.