If you’re completely serious, just google your question and click virtually any link. Most of them will work, and be “legitimate”.
Buying gil causes a number of problems, both for you, and for your server, though.
First off, FF14 Gil farmers don’t tend to earn their cash legitimately. See, farming the kind of cash they need to make an actual profit requires a relatively large-scale, high-in-game profit operation that runs across every server in the game. So, individuals who fund these gil sellers by buying their product encourage their practices, which include the use of farm-bots and programs that spoof the servers and allow them to manipulate dungeon instances to perform things like teleportation or short-term invulnerability. I can’t say with certainty that this is how things are done now, but roughly a year ago, the sellers were using teams of five black mages to run Wanderer’s using bots and a spoofer to teleport past gates and avoid most of the trash mobs.
This approach to earning gil doesn’t stimulate the economy, but rather continually adds new gil into the game which can then flood the economy. Coupled with farmers that use bots to farm oft-ignored and very profitable crafting materials, they can adversely impact the cost of items on the marketboard, either by driving competition on valuable items way up (and consequently, prices way down) or by flooding the market with excessive amounts of FFXIV Gil, which can subsequently see an inflation in all prices, which makes items harder for honest players who don’t spend much time working the economy (like myself) to afford.